There are two box 52s & two box 53s on the 2024 T3. One set is for before June 25, 2025, and one set is for after June 25, 2025. The sum of Total Capital Gains box 52 for before June 2024 and Total Capital Gains box 53 for after June 2024 have to equal to what is in box 21. And your Foreign Capital Gains can't be more than your Total Gains.
When you enter your Foreign Capital Gains, you have to make sure it goes in the box that matches the period - before or after June 25. If you put a Total Capital Gains amount in box 52 (before June 2024) but then put Foreign Capital Gains in Box 53 (after June 2024), you'll get an error because your Foreign Capital Gains for that period is higher than your Total Capital Gains for that period.