There is a way to be able to play any level at will in Sonic Mania.
Sonic Mania has multiple fantastic levels, some old, some new spins on older ones, and some all new- but you aren’t done with them just because you finished playing them. You can go back and play them at will, any time, thanks to the secret level selector in the game.
This level selector exists in addition to one that will show up after you complete Green Hill Zone Act 2, at which point you have the choice to go back and replay any level you have already completed to participate in time trials. This one, you access via nifty cheats.
PS4 AND XBOX ONE