Question

he wrote in the sand because wind can erase it way

Answer 1

"He wrote on the sand because wind can erase it way "

- This is the correct sentence

Answer 2

Actually ' in ' is not the correct preposition here.
Thus, the correct answer is :
He wrote on the sand because wind can erase it away.
Here, we can only write on sand. But ' in ' can be only used to describe those which are within or inside. So we can't write within the sand that's wrong.
Hope u understand!