Did you download and install the 64 bit version of Garritan?

Thanks for replying.  I'm not sure what (64 bit of Garrtian) that is because there is nothing on the site (that I could find) that indicates it is a unique 64 bit “Garritan” installer. But the first thing I tried a few days ago was to download and install what I think is the most current Aria Player (v. 1.959) which given its year of distribution (2018) I can only assume is 64 bit. Then, this morning I purchased and installed GPO5 and because it was just bought I’m assuming that it contains a 64 bit installer. (Is that what you mean?) Also, after it was installed I didn’t get an error message that says it’s in compatible with Catalina (and Catalina will not play 32 bit applications) which would happen if it were not 64 bit.  And again, I bought it this morning and I still have the same error message. When I delated GPO4 I followed MakeMusic website and deleted all the files marked within the library/application support and the library/preferences -- I deleted the P lists, I deleted the applications. I don’t know what else to do. (BTW, sorry for the all the non-sensical typos in the prior post -- typing quickly without glasses....).  But no matter what I do, somewhere, somehow I keep getting this error message.

ARIA Engine: Personal Orchestra Personal Orchestra 4 -- Cannot find instrument bank file at volumes/Gtech/Virtual Instruments/Garritan/Garritan Personal Orchestra 4/Banks/Standard/GPO/FLT.bank.xml. Do you want to unregister this product to make this message go away?

Cannot find instrument bank file at volumes/Gtech/Virtual Instruments/Garritan/Garritan Personal Orchestra 4/Banks/Standard/GPO/FLT.bank.xml.

Is the GPO 4 bank file at that location, or not? The location that ARIA looks for the libraries is stored in the file at: /Library/Preferences/com.Garritan.Personal Orchestra 4.plist (or similar). NB: this is the root level Library, not the user library.

If you've bought GPO5, then I believe everything from GPO4 is included, so you don't need both. If you've deleted everything else, then why not click "Yes" to unregister the product?

Good morning Ben, 

I think I'm looking in the right place: In finger, under GO, hold down option key, then LIBRARY appears, then to the next level, then scroll down to PREFERENCES. I don't have any com.Garritan.Personal Orchestra Plist, nor do I have any com. Garritan files of any kind (nor com.Aria or com.MakeMusic.). Yet somehow I'm still getting that error message. Anywhere else to look?

That's the User Library. Try the Library directly under Macintosh HD.

(Type Command Shift G, then type /Library and press Return. In fact, you can just type /Lib then TAB and it will auto-fill the rest.)

Here's an interesting development -- I managed (I think) to delete as much as I could find. I re-installed Aria Player v. 1.9559 (which is 10.15 compatible). When installing, I de-selected the vst and vst3 options (because I only use audio units). It installed and when I launched Aria Player this time I rep'd the error message "Cannot find aria engine! Please re-install". So I did. This time I didn't deflect anything and loaded all options (onto system drive). Then when I opened it I got the same "Cannot find instrument bank file at volumes/Gtech/Virtual Instruments/Garritan/Garritan Personal Orchestra 4/Banks/Standard/GPO/FLT.bank.xml." 

This is maddening. And I've written to MakeMusic twice. I own 4 of their products (Instant Orchestra, GPO4, GPO5, and World Instruments". Not a peep from them

When you install GArritan stuff, I think you get asked where you want to install the samples. There is a support help page about moving the location of the samples.

You still haven't said whether the FLT.bank.xml file is at that location, or not. But as I also said, if you have GPO5 working, then you don't need GPO4 as well. 

I concur that MM have been very quiet of late. I know these are difficult times, but you'd think that someone could 'work from home' and support the forum. 

Okay. I just had some success! After deleting everything I as FINALLY able to open ARIA player (oh, and to answer your previous question about why I didn't just say "yes" to deregistering as if I clicked yes, the box would simply re-appear with same choice. When I clicked no, it would just re-appear with the same voice...). So here is where I now am -- I can open the standalone Aria player on my Mac but no Garritan products are showing (in the upped left it should say "GPO5" and other instruments. When I open Logic Pro X, same thing -- I can now choose Aria as the player but no Garritan product names (e.g. GPO5) appear in the upper left so no libraries are seen. How do I add my Garritan libraries (Instant Orchestra. World Instruments, GPO5 to the player? Normally they should should appear but not in this case.

OK, in /Library/Preferences, there is a file called com.plogue.aria.plist. This contains the installed banks that ARIA knows about. Each of those, then has its own plist, e.g. com.garritan.Instruments for Finale.plist, which describes the location of the sfz instrument files and the audio samples. 

Clearly, your setup has got into a tangle between where things are and where ARIA thinks they are.

Success! Yes, this has been a journey. I've stayed on a Mac running 10.8.5 and LP9 for years too long because I was dreading the change over. When I first started getting into music Logic and software instruments were very complicated when it came to installing. It's obviously much better now especially because of these forums so you're not sitting in a vacuum pulling your hair out. Community effort. Thanks for your assistance!