Other people have had similar issues with some Mendix versions, maybe one of their solutions will work for you:
you can find it here :
Hello, Rola Muhareb
If you are unable to find a sign-in page then create a new one by right-clicking on my-first-module then click on add page then go to the login page template and select one default page template and click on ok. Then add this page to your navigation.
I hope this will help you.