Internal users with Azure Active Directory credentials trying to sign-in without a valid license assigned will see this message on the sign-in page: You don't have a valid license to access this website. Contact your system administrator.
Users could also see this error if Pay-as-you-go hasn't been enabled for this environment.
Depending on the purchased SKU type - such as different Dynamics 365 SKUs, Power Apps per app plan or Power Apps per user plan, an administrator will need to either provide an appropriate license to the users, or have appropriate number of app passes available to the environment. For more information about app passes, go to Allocate or change capacity in an environment. Once app passes are allocated to the environment, the website will have to be restarted for changes to take effect.
For details about licensing of internal users, go to Power Pages Licensing FAQ.
For details about licensing of internal users using Pay-as-you-go, go to Power Apps per app meter.
For details about use rights to Power Pages for internal users, go to FAQ use rights to Power Pages for internal users and Licensing Guide section Power Pages use rights included with Power Apps and Dynamics 365 licenses.