This message will appear if you have entered the "months" in the wrong box for the type of benefits you are receiving. You will need to go back to the T4A(P) - Canada Pension Plan Benefits slip page and ensure your entries are correct.
To do this:
This should remove your "Error" message and you should be able to NetFile your return.
I tried everything deleted and reenter and still say fix error due to months not added and it is included