You may want to file an issue on Castersoundboard’s GitHub here (and then link to your Issue here) but it seems to be @wimpress who made this snap so he may have an idea of how to fix this
Hehe… it was Wimpy who told me to post it here last night. 
Said it needed greater minds to be engaged. From what I gather though it is not isolated to castersoundboard snap. As you might also see on the thread about warzone2100 and Krita something is falling back to the legacy ubuntu-core dependency even though that was deprecated a few months back.
Is there some way to audit the listed snaps to see if any contain impossible build dependencies. What I find irritating is they install fine so don’t complain about dependencies then. But they fail to run requiring ubuntu-core.
A little odd, don’t you think?
Anyway, not that their is anything I can particularly do about it as it will need to be fixed further upstream but seems there is a lack of clarity over the issue which some snap maintainers might not realise as being important.
Thanks.