Please check again and post a new question. Peace dollars were minted from 1921 to 1935.
The is no 1921S Peace dollar; Peace dollars were not produced in San Francisco ("S" mintmark) in 1921. If your 1921 dollar coin is a Peace dollar, it was produced in Philadelphia and has no mintmark; if it has an "S" mintmark, it is a Morgan dollar produced in San Francisco).
The 1928 Philadelphia Peace Dollar has no mint mark, indicating it was minted in Philadelphia. A normal 1928 Peace Dollar would have a D or S mint mark denoting it was minted in Denver or San Francisco. The absence of a mint mark on the coin indicates it is the Philadelphia Mint version.
1924 Peace dollars were struck at Philadelphia and San Francisco
No Mintmark just means it was made in Philadelphia.
There is no mint mark on the 1921 Peace dollar because they were all minted in Philadelphia and thus carry no mint mark.
1922 Peace dollars were minted in Philadelphia, Denver, and San Francisco.
The mintmark on a Peace dollar is located below the word ONE on the back of the coin. Keep in mind that if it was made at the Philadelphia mint it won't have a mintmark.
Coins from the Philadelphia mint had no mintmark before 1980, except for the wartime nickels.
This is a Peace dollar. The mintmark for Peace dollars is located on the back to the left of the eagles wingtip. Keep in mind it may be blank if it was made at the Philadelphia mint.
Please check your coin again. 1921-dated Peace dollars were only struck at Philadelphia. If your coin has a "D" above the "DO" in "DOLLAR" and the eagle's wings are outstretched, you have a Morgan dollar rather than a Peace dollar. There's more information at the question "What is the value of a 1921 Morgan silver dollar?"
It's on the back below the ONE no mark means Philadelphia.
The Philadelphia issue (no mintmark) in the circulated grades is valued at $260.00-$440.00.