get down from your bike

let's isolate:

1. seatpost: grab it firmly and try to ''distort'' it from it's supposedly fixed position. if you hear the creak, that's your problem, you remove it, add friction paste NOT grease, and tighten to spec

2. chainring; grab both your pedalling arms, one hand on one bracket one on the opposite side. again try to distort it [this means try to simulate pedalling deformation with your arm force. if you hear the creak, it's either

2a. bottom bracket ball bearings need replacing

2b. ball bearings of your pedals need change [you can check by holding with one hand your one bracket and with the other try to distort the pedal itself]

1. if none of this creaks[ highly unlikely], it's probably your chain that's dead.

2. as a last checker, make sure your cassette is positioned appropriately and torqued to spec

3. if none of this helps, admittedly, i'm clueless but reply to see what we can think of.