Just to check understanding here.
You have a Project that is saved pointing to a specific 'database' and its model.
You open the project and attempt to select a different database and then get the error message.
Can we assume that the basics have been checked - access rights, correct name (selected from a browse session or verified as types without typos), that the alternate database is indeed exactly identical, other than name (Field definitions, names and 'Names'), not open and locked in another application.
Can the other file be opened in another Monarch session skipping the Project?
Mostly basic stuff I'm afraid, but I have been caught out enough times by not checking such things accurately before diving off looking for a complex answer to a complex problem that I hope you will foregive me offering a basic tick list here.