Same here, most likely I will not use TurboTax anymore.

Box 21 on the T3 for 2024 is just the sum of the amounts in boxes 52 & 53. You need to put whatever is in Box 21 in either 52 or 53 for it to transfer to Schedule 3. It doesn't matter which as they are both taxed at the 50% rate, but for some reason using just Box 53 seems to work better for some people.

Tried both ways, filling only box 52 or 53, TurboTax still give a warning and not let fill the taxes online.

Very frustrating because the issues is known not not fixed.  Spend already hours for same issues, looking online, started again by deleting everything and filled again. 

Looking for different tax software and return TurboTax box back to Costco.