If you entered a T3 slip in your return with an amount in Box 24, you must specify the province of Self-Employment even though you do not have a business. You can click "Fix" and "Applies to Me" on this error message, then enter your own province of residence as the province of self employment.
The CRA considers amounts in Box 24 of the T3 as business income and those amounts are transferred over to Line 135 of the T1 General Tax Return.