I don't have any experience with the ForScore app itself. They do have a user guide, and that would be the first place I would look.
forscore.co
If that does not work, and you are just trying to merge PDFs into a single file, there are several options. You would need to use them before importing into ForScore.
Here are the two I tend to use.
Most of the better PDF editing apps will allow you to combine, add, delete, or rearrange sheets in a PDF file. I like Documents by Readdle, because it has some extra file managment features, but PDF Express by the same company has all the same editing features. With the new files app, the managment features in Documents are as important as they used to be. Other's may suggest their own favorite apps. Some are probably free, but I haven't been keeping up with all the many many apps out there.
You can create shortcuts using the Shortcuts app. This a little more involved, but if you like that sort of thing you can tweak the way it works to your own style. There is one app in the Gallery section that would do the job easily enough.
You can either write them to open a file selection tool and combine the selected files, or you can use the files app to select files then run the shortcut to do the job.