Translation:The mosque is located in the city centre.
The mosque is located at the centre of the city
This should be accepted. 'At the' or 'in the' are both acceptable in English.
I'm assuming this is not accepted because the sentence was only added three months ago and there hasn't been time to add all of the acceptable answer permutations.
I know it's tempting to use "is found" because trouver means to find and it still makes sense in English, but se trouver is a different verb. It is reflexive and it means "is situated," or "is located."