The error message "invalid S/L or T/P" happens when:

Incorrect placement:
The stop loss (S/L) or take profit (T/P) level is set too close to the current price, violating the minimum distance requirements.

Wrong side:
The stop loss or take profit is placed incorrectly:

• For a buy order, the stop loss should be below the current price and the take profit should be above it.
• For a sell order, the stop loss should be above the current price and the take profit should be below it.