Hi, @Marshy!
While the detailed error does not help much to explain what is going on, can you please confirm how you attempted to edit / delete the message and where did you see the error you shared? Was it done from the desktop / web app like this?
Are you using E10 / E20 license? If yes, can you please confirm how the permission scheme looks like on your end? If you are the only user and owner, you should be assigned to the System Administrator role which should look like this:
Keep me posted. Thanks.