Tight end use on roster

Can a tight end be used in flex position on active roster in the NFL fantasy game ?

Absolutely! If you have a good enough TE, and you’re in a pinch for a starter, go for it. Or if your backup TE is facing a more favorable matchup than your backup RB’s and WR’s, maybe squeak the TE into the flex.

If you’re asking if you should do it regularly, I’d advise against it. If your backup TE is your best option every week, then you need a new RB or WR.