What can I do to ensure my GTT order (Custom Validity Limit Order) is getting placed successfully on Upstox?

To ensure your GTT order  (Custom Validity Limit Order) gets placed successfully on Upstox, ensure you have sufficient fund balance in your Upstox account for that GTT order to be processed.

If I had to ensure fund balance, what’s the point of GTT order  (Custom Validity Limit Order)?

We totally agree with you. These are supposed to be effortless, but with a monthly auto-settlement process mandated by the Exchange, any positive fund balance gets auto credited to your bank account. This leads to your fund balance becoming zero. That’s why GTT orders could fail despite your order hitting its price.