I am very scared as I have seen some posts of people getting banned for single player cheat engines.
No, you cannot be banned just for having software downloaded.
Having it in background?
Technically they could ban you for that but it is extremely unlikely as that opens them up to lawsuits.
https://www.reddit.com/r/FortNiteBR/comments/8y9dx9/banned_for_cheat_engine_when_it_wasnt_even/ Saw it on reddit so yeah. Was like 2 years ago though things may have changed.