Check to see if the Account was created for the GC, and the balance of that Account. The GC Payment Type will only be enabled if the associated Account has enough funds. Note that this should match the GCBalance field of the GC Entity, but if it does not, then something went wrong during GC Account creation. The Payment type is tied to the Account, not the Entity itself.
Also check the GC Account Type Rules to ensure it is set to “Credit”.
If the Entity is not created, then the Account is probably not created either. But even if the Entity is created, but the Account creations fails, then you cannot use the GC as a Payment Type, because the Account does not exist, thus has no Credit Balance.
For the GC Entity Screen, it is set to filter to show only GC Entities which have an Entity State (GCStatus) of “Purchased”. Again, the GCBalance shown there should match the Credit amount in the GC Account, but using it as Payment is not contingent on the GCBalance Entity Custom Data Field (it is simply a visual indicator on the GC Entity Search Screen); rather the GC Payment Type is contingent on the GC Account Balance having a Credit (shown as a negative value in brackets, like (15.00)) which is greater than or equal to the Tendered Amount.
If you cannot create GCs any longer, then you have messed up the Entity/Account creation flow.