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