The Date To field on the provider contract screen in Admin can' be edited directly. This is by design, to close a contract, you should use the Terminate Contract functionality rather than entering an end date manually.
Terminate a provider contract
Go to Payments - Contracts - Provider Contracts menu option in Admin.
Right-click on the contract you want to terminate.
Select Terminate.
đNote: If you have imported service orders into Admin that are linked to the contract and they are still open, you will not be able to terminate the contract.