License restrictions on permission set assignment
Key Takeaway
A permission set can only grant permissions that the user's licenses support. When the user license doesn't include a feature, the user needs both a permission set license and a permission set that contains the feature permissions.
Licenses limit what a permission set can give a user. Knowing how user licenses, permission set licenses and permission sets fit together explains when a permission set assignment works and why it fails.
How licenses limit permission sets
Every Salesforce user has a user license. A user's assigned licenses define the maximum functionality available to them, so they act as a ceiling on what the user can ever be allowed to do.
Permission sets add permissions on top of a profile, so they extend a user's access without changing the profile. However, permission sets can only hand out permissions inside the ceiling. To grant a permission through a permission set or a permission set group, the user must have a user license or permission set license that supports it. If you try to assign a permission set to users who lack the required licenses, you get an assignment error.