Hi,
Yes, that's normal, you need to check all the users groups in that setting of the variant. If not all of them are checked, the system requires a user account for the customers and thus guest users can't checkout.
That's because it is impossible for the user group after purchase plugin to change the user group of a user which doesn't have a user account.
Normally, all the user groups are checked by default in that setting, so that shouldn't be a problem normally.
But yes, if you don't use the plugin, it's best to leave it deactivated anyway. That will be less memory used, less processing done and less probability of an issue in the future.