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