The "edit_roles_grantable" capability will only allow the user to create/edit the role if they have listed the roles in "Inheritance" section on the custom role.
For example, if you want to create/edit a power role, the user must at least be assigned a custom role which inherits another custom role which has power capabilities or the power role itself (as shown below).
Once the user logs in and attempts to create a new role, they will be able to only select from the following list.
However, if you would like the user to be able to inherit from all available roles, you can add the "edit_roles" capability to achieve this. Documented here: