The taxable capital employed in Canada is used to determine the corporation's eligibility as a small business, and is for information purposes only. It is drawn from last year's Schedule 33, Line 690 (roughly equivalent to the Retained Earnings), and will only have an impact when it exceeds $10 million.
For the purpose of completing the return, if the value is zero, you can sign it off as such, or enter 1 to remove the warning entirely.
The Taxable Capital employed in Canada for the previous taxation year can in some situations just be the retained earnings for the year. There are also other amounts that have to be considered in the calculation such as contributed surplus and loans and advances to the corporation. If you go to Schedule 33 line 690, you will see the calculation of what is included and what is deducted for Taxable Capital Employed in Canada.