This message is actually not as severe as it sounds, the Master.ins file simply has instrument definitions for various hardware synthesizers.

This can be corrected by holding down the SHIFT key on your keyboard when launching your Cakewalk software. This should prompt you with a message that says “... will now personalize your user settings...”. Make sure to click [OK] to this message.

Afterwards, it should copy over the Master.ins file behind the scenes and you will not see this message again.