Please ensure that you have sufficient margin in your account to cover the trade you’re attempting to place.
Next, check that you haven’t entered your stop-loss or take-profit levels too close to the current market price.
Finally, if you have checked the maximum deviation box, perhaps the market price moved by more than the pip-range you specified, in which case you may have received a re-quote. Increasing the pip-range in the deviation box can help increase the chances that your order gets filled even during volatile market conditions.