I am running Scapple 1.5.3 on macOS Tahoe and it crashes reliably every time I launch it.
I uninstalled it and downloaded a fresh copy from L&L download page after uninstalling my current version and it made no difference. I have been having this issue for a few months now. I am finally getting a round to contacting you. I like this app and I would like to use it.
In the meantime, can I suggest moving the Scapple preferences file out of Library > Preferences to somewhere like the Desktop and then trying Scapple again.
If that sorts it, then you’ll need to reset all your preference settings; if it doesn’t, then just trash the new one which will have been created and move the original one back, ready for when the team have sorted you out.