What it means
The platform will not accept the stop-loss (S/L) or take-profit (T/P) price you entered. Something about the level breaks the broker's rules for that symbol, so the whole order is refused.
The order itself may be fine — it is only the protective level that is wrong.
Common causes
The most common cause is placing the stop or target too close to the current price. Every symbol has a minimum 'stops level' (a minimum distance in points) that your S/L and T/P must respect.
Others: putting the level on the wrong side (a stop-loss above entry on a buy, or below entry on a sell); entering a price with too many or too few decimals; or trying to modify a level while the market has moved so your once-valid price is now too close.
Step-by-step fix
Right-click the symbol in Market Watch, open 'Specification', and read the 'Stops level' value. Make sure your S/L and T/P are at least that many points away from the current price.
Check the direction: on a buy, the stop-loss sits below your entry and the take-profit above; on a sell, the reverse. Re-enter the price with the correct number of digits for that instrument, then submit again.
How to prevent it
Give your stop and target a little breathing room beyond the minimum distance. If you trade very tight setups, choose instruments and brokers with a small or zero stops level. When modifying an open trade, remember price keeps moving, so leave a margin.
Broker's fault or yours?
This is a rules issue, not a fault. The broker sets a stops level partly for legitimate reasons around fast markets. It only becomes a concern if the required distance is unusually large, which makes tight risk management awkward — worth comparing across brokers if it affects your style.