… What can I do to open my Finale 2011 files and then convert them to xml files …

Perhaps go back to Mac OS Sequoia ?

Maybe update to F27 if still possible (my understanding is that F27 is no longer being sold, in lieu of Dorico, but there may be copies out there on eBay etc).

F2011 is 25 years old and it's pretty unrealistic to assume it will still work with a more recent version of an OS. And keep in mind that Tahoe will give way to the new macOS release in the coming months (it's already in testing). As I've reported here and elsewhere, along with others, there are many issues with trying to run F27 in macOS Tahoe. I switched to Dorico two years ago when the death of Finale was formally announced (and let's be real; it was circling the drain for about two years with little or no improvements or enhancements). Other than the occasional tweak to an old score, I no longer use Finale and suspect it will be less and less usable over the coming 1-3 years. The sad thing is that .xml to Dorico or Sibelius or MuseScore will only get one so far. I know this isn't at all helpful advice, but it is the reality. I spent the last year making sure my scores were all in decent shape and fixed some minor things in Finale, despite the many things like SmartShapes that are really borked on my system. I am not sure I could really make a lot of parts from my old scores, but then again, no one is beating down my door requesting parts in order to perform my stuff. 

Do you at least have PDFs? 

 I can't think of a Mac model that can run Tahoe and also run an OS capable of supporting Finale 2011. Did you get a new Mac? Do you not still have the old one?

I presume you didn't get a copy of F27...?

You can still download F27 here:
https://store.makemusic.com/Downloads/Default.aspx?id=674

Though you'll have to use it in demo mode, so you've got 30 days to convert all your files. 

Otherwise, you'll have to find someone else with Finale who's prepared to do it for you. (I have to say, there has been some indication that action is required over the last two years...)

No idea how you were running Finale 2011 on a macOS newer than 10.11 El Capitan The only way to get there was if installed over Mavericks or earlier and the OS upgraded. 

MakeMusic upgraded me to Finale 2014.5 for free so that I could help people running 32bit macOS through Mojave. Macs built 2012 through 2018 can run F2014.5. Those built later cannot.

Although, as Ben points out, you can download Finale 27, it only works for 30 days without a license but that gives you plenty of time to convert your 2011 files to MusicXML. I know that there's a secondary market for licenses but I don't know if the MakeMusic License Server will accept new registrations. It might—that server would need an upgrade that I doubt will ever happen.

The announcement went out July 24 that the crossgrade to Dorico no longer includes a license to F27 but the storefront still says that it does. It knows that I have four upgradable licenses, however, and two of them are for 27 (other two are Finale and PrintMusic 2014.5). In any case, if you've converted your 2011 files to MusicXML 4, it will be much easier to open them in Dorico.

If your 2011 files open blank in 27, you will need to change the display fonts to see them. I recommend Maestro.