Here is the full list of all 18 Sonic Mania achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Wreak havoc at the propaganda factory
That elusive perfect run, only a professional can achieve
Gotta gacha 'em all
You might have to submerge to find it
We really like boats
Get through Stardust Speedway Zone as quickly as possible
Have your photos taken in Studiopolis Zone
Collect gold medallions in Blue Spheres Bonus stage
Find the hidden item boxes at the end of the Zone
Try for a 3 chain combo!
Collect silver medallions in Blue Spheres Bonus stage
Let the wind take you through
Collect all seven Chaos Emeralds
Achieve any ending
Barrel through the lava, don't let anything stop you
Try pushing a barrel to see how far it goes
What would happen if you cross a bridge with a fire shield?
Spin the Star Post!