That's a Mac OS error and can mean anything. Normally, all it means that the file or library cannot be found.

We need a lot more information.

https://makemusic.zendesk.com/hc/en-us/community/posts/215729367-General-Discussion-House-Rules

Try opening Finale and then importing the XML via the File menu.

(Not sure why your Finale 25 file is an XML; F26 can open a .musx file from F25.)

I'm such a dummy, I never tried opening the v25 file by double-clicking on it.  It opened in v26, I saved it, & voila, I'm back in business.  It was trying your suggestion that led me down this path - thank you!