there is nothing wrong with your T3.  I am having the same problem.  Box21 is total capital gains.  Box 52 and 53 need to add up to Box 21.  Its a error in the software where box21 is not being transferred to the corresponding line 68140. 

There is another post on this but no fix from Intuit.

I have a filled in box 21 but nothing in the footnotes about box 52 and 53.  I don't know if this is a t3 issue, a cra issue or a TurboTax issue.  All I know is I have a t3 issue on a tax law that didn't go through so this should be a non issue.  Make 52 + 53 = 21!!! And ignore period 1 and 2

Can you please confirm if you imported your T3 with our AFR? If yes, can you please go on your CRA My Account and verify your T3 to make sure that each box on your T3 was entered when imported?

I did not download the T3.  All entries done manually.