Unable to create customized mobs using commands

When I try to create a custom entity using commands, the output shows as ‘unable to summon entity’. Something as simple as having a custom name is not working, and still shows up as ‘unable to summon entity’. I am not able to do it in any world, including Realms. I have tried restarting and uninstalling and reinstalling and everything, but it doesn’t seem to have fixed the problem. I think I have tried everything, so I made a post here to see if anybody else is having this problem?