Standard versus custom profile editability
Key Takeaway
You can't edit a standard profile's permissions. You also can't change a profile's user license, even on a custom profile. So to get an editable profile for a user license, clone a profile that already has that license and then edit the clone.
A profile sets a user's default access and settings. Every user gets one. Every org includes standard profiles, but you can change only a few of their settings. A custom profile, usually made by cloning an existing profile, lets you change almost everything. Knowing what each type allows tells you when to clone a profile and when to use a permission set instead.
Profiles and user licenses
Profiles define default settings for users. You assign a profile to each user when you create the user. Every profile belongs to exactly one user license type.