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