This sounds like you translated verbatim from Russian. One can understand, but the underlying grammar is Russian. It would be better to say: what her motivations for stealing the watermelon were or what motive she had had for stealing the melon Hope this helps, if you need more help please feel free to reach out

Hi, Trying to use a relative clause for the object doesn’t sound very natural here; better to just use the object and a prepositional phrase. “I really want to know her motivations for stealing the watermelon.”

Hasmik, Kacie Collin, thanks a lot.

The grammar is ok, but if you wish to have a better and more correct version you can say: I really want to know, what type of motivation she had... or: I really want to know what type of intent she had... Hope it helps :)

"I would really like to know - what were her motivations for stealing the watermelon?" OR "I would like to know what her motivations were for stealing the watermelon."