Double click the error and you end up on the page with the error. Now look at the properties and (re) select the allowedd user roles. Just opening the select screen would be enough to remove the error.
The issue is that you probably had a module role called user in the module MyFirstModule. You allowed this module role access to a few pages and later on deleted the module role “User”. Hence all such pages are throwing up this error. As Ronald has pointed out, when you will double click on each error, you will be taken to the corresponding page, you need to remove this Module role from “Visible To” property of the page and allow other module roles that you might have.
thank you, the errors were fixed.