is this word order wrong? "Sub pavimento rapide aqua fluit"
No, although the adverb is usually right in front of the relevant verb: rapidē fluit.
I moved around the word order, to achieve the surprise (as I thought it) of water being under the floor:
Sub pavīmentō rapidē fluit aqua.
But it wasn't accepted.
I got marked wrong for putting the subject at the end of the sentence. Sub pavimento rapide fluit aqua. That word order should be allowed. .
Latin is typically SOV (subject object verb).
Omnia vincit amor.
... et nōs cēdāmus amōrī !!
Expeianced handyman here, sounds like a burst waterline.