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