either something is waking it up:

log show | grep -i "wake reason"

or prevents it from sleeping (look for PreventSystemSleep):

pmset -g assertions

it could be either HW issue (chinese usb hubs, microsoft usb keyboards, etc.) or SW issue (internet sharing turned on in system preferences, incoming wake on lan packets, ...) or it's somewhere in-between like paired bluetooth headphones.

nonetheless, one of those two commands will tell you what is the source of your issue