Field Security error

I’m getting this error when trying to remove users/groups from the Customer table and the CustomerType field in Field Security Maintenance.

I’ve been trying to use native Epicor screens to accomplish “read-only” fields but also have a specific user/group to be able to change the field… It’s looking more and more like I need to add code to do this - even though there’s a screen that should do exactly what I’m looking for

Here’s the error-

Edit - I noticed I didn’t put down exactly what I’m trying to accomplish.

We’ve had issues in the past with our users entering in duplicate customers and what not - so we would like to make it so only a set group of users can change/update the CustomerType, ValidSoldTo, and ValidShipTo fields. I figured I could use Field Security and just go into each of these field and delete everyone except for the set group of users - but as you can see from the error; it is not working

Found out that we can’t delete anyone out of the list- but we can make them Read / None access.

Why give the ability to delete rows if it’s just going to cause issues?

Either way - I have accomplished what I needed by just giving every user Read access, and the few that can change the fields; Full access