It would help to see an exact image of the error message you are receiving. Otherwise we are just guessing in the dark and will probably waste our time and yours.

As far as 27 is concerned, 10.14 is the minimum, so it won't work reliably in High Sierra (10.13). If I remember correctly, this message appears when there is a compatibility issue.

The other versions should work in High Sierra, though. Submit a request to Support.

At least, you found a solution restarting the Mac as a guest user. 

The trouble started after I installed a full version of Band In The Box. That app has no issues, but did crash yesterday for the first time. I'm guessing BIAB and Finale use some similar things. mI noticed that if I run Finale my version of Encore gets really fouled up too. Every app must be using some MIDII resource I'm guessing. I'm afraid to upgrade my OS, but may have to do this too. Can't figure it out here on my end.

Check your permissions. If it works under the guest logon, it might be a permissions thing. (Windows anyway, don't know about Mac)

I am running BIAB Audiophile and 27 over Catalina with no conflict. If I boot up with my Mojave drive, Encore works also—BTW, much better than High Sierra but Mojave’s the last MacOS it works over.

Unless you have a 2011 or earlier iMac, it’s time to upgrade to Mojave. If your Mac is 2011 or older, time to upgrade your machine to a 2017 iMac or iMac Pro if you need to still use Encore. The 2019 iMac will run Mojave but cannot run High Sierra which might make transition difficult. You cannot run Encore on a 2020 Mac.

I guess I'll try a Mojave upgrade to start. Would love to go to Big Sur in one jump, but I'm afraid of all the upgrade issues, so a step at a time may be the way to go. I purchased my iMac in 2018 (April).

>I guess I'll try a Mojave upgrade to start. Would love to go to Big Sur in one jump, but I'm afraid of all the upgrade issues, so a step at a time may be the way to go. I purchased my iMac in 2018 (April).<

If there's a chance you are running any other 32 bit apps, you do not want to go to MacOS 11 till you are ready but you do want to go to Mojave.

With a 2017 (if you purchased new in 2018, that's what it is), it's easy to set up an external SSD as a Mojave boot drive — then you can make the jump to Big Sur on the System drive and you have the other if you need any of those apps. 

Since Encore cannot export lyrics or expressions via MusicXML (only notes and text), I converted all my files to MusicXML and printed to .pdf so that I can import or scan as needed. Through word of mouth, people have learned that I can do this which is the main reason I keep a Mojave boot drive. Users who went to Catalina or Big Sur without doing that first are out of luck otherwise. I never do any work in Encore anymore even though it is a little faster for my church work than any other notation app.

Don't know how long you've been a Mac user but there may be other things to consider. There are apps that convert old WP, database and spreadsheets going back to MacWrite and earlier and many are free.

If, however, you have old graphics apps such as MacPaint, MacDraw or the graphics engines in ClarisWorks or AppleWorks, There is an app that can convert these old files to 64 bit—you must use the 32 bit version in Mojave or earlier to do the converting and then the files are available in the 64 bit version that is compatible with Monterey. A purchase of the 64 bit version entitles you to a free license for the 32 bit app on request.

https://www.eazydraw.com/ 

Anyway, before I upgraded to Catalina, I was converting Encore and graphics files for a few days. Then I was upgrading software licenses. Let's just say it took me awhile.