If you find the checkbox on the Check for Payment Printing window is grayed out, here's why:
The house bank check printing has been locked. The lock checks printing feature is to prevent simultaneous printing and/or updating the checks for the same account by more than one user. This feature also avoids printing and or confirming checks using the same check number.
How to Fix
Under the main menu, go to Administration > Setup > Banking > House Bank Accounts. In the House Bank Accounts - Setup window, deselect the Lock Checks Printing checkbox for the relevant bank accounts. You will be able to print the check now.
Note: The Lock Checks Printing column is only visible to super users.
For more information, see SAP troubleshooting note. (Login required using your S-number and password)