Mob wont spawn with spawn egg?

As the name suggests, the spawn egg fails to spawn in the custom entity, and when I use the /summon command, it says:

Unable to summon entity

I'm honestly not sure what the problem with it is.

I can provide screenshots if necessary.

I'm using the plugin "Fabric Loader ", and the version is 1.20.1. I have also tried exporting it as a .jar and trying it and it has the same error.