It might help if you said which version of Scapple you have installed, and which version of MacOS? For instance, if you’ve just upgraded to Big Sur, but are running an older version of Scapple, you can expect problems … you need to install the latest version.
I’m sorry I can’t help you further. Although I have Scapple, I’ve never really used it.
However, even worse is the fact that my 2011 MBP running High Sierra, on which I have Scapple, stopped working on Friday night, so I can’t even play around to see if I experience the same.( A search suggests that it is the PRAM battery which has failed … I sincerely hope so because that should be repairable, otherwise my bank account is going to take a very big hit!)
But someone else who has more experience of Scapple will come along soon, I hope,