Looks like you might have deleted the module role User in MyFirstModule, but it is still assigned to either a microflow, page or application wide user role.
Double clicking the error will usually point you towards the Microflow of Page with the issue. In this case, you can re-select the user roles under allowed roles in the properties window.
Lastly, you can check the security tab under under project in the project explorer. You might have a user role that is assigned to the User module role from MyFirstModule, which has been deleted. In that case, you might need to re-select the roles there.
Easiest fix would probably be to re-create the User module role in MyFirstModule -> Security. Was there a reason you wanted to delete it?
Have a look at this for more information:
It seems you removed the user role User from your MyFirstModule. You can have user roles defined on the project level and on the module level. Open the security in MyFirstModule and add the role again or in the project security change what module role a project role has. So let's say you have on project level admin, manager and user. On the module level you have admin and user. Then on the project level you can set which role a role on project levels has on module level. So a manager on project level can either have the user or the admin role on module level.
See also this learning path: https://gettingstarted.mendixcloud.com/link/module/96/lecture/788
Faced the same/a similar issue in verison 9.13.1, solved it the following way:
There were still entity access rules set to “User”, after I changed its name. So to make those access rules visible within the entities’ properties in the domain model, I had to change the Security level in the App Security to Production. Then I deleted all the access rules from the respective entities, changed the level back to Prototype/demo and the errors disappeared. If you are already in Production level from the beginning, you don’t need to make that change. If not, don’t worry because of the many appearing errors because of the level change in-between, they will disappear later again.
Hope that will help anyone.