The most common cause of this is that the Tax Status of the CIS record is set to Unmatched.
In order to pay CIS subcontrators in the software, the Tax Status of the CIS record for the contractor must have been set accordingly.
Please check your sub-contractor (CIS) record:
Select Creditors, Suppliers (ribbon menu) or Purchase Ledger, Records, Suppliers (classic/module menu)
Find the relevant supplier record
Click CIS (ribbon menu) or the Subcontractor icon (classic/module menu)
Check the setting for Tax Status - if this is set to Unmatched the system will not allow you to pay the sub-contractor (the Tax Status is how the system calculates the relevant tax (Gross, Standard or Higher)
Ensure that this is set to Matched.