Box 12 A is blank on my w2, Turbo tax keeps asking me to review my return and marking the box as "needs info"

Double-check the amounts you posted in Box 3 and 4.
Box 4, Social Security withheld, should be .062 (6.2%) of Box 3.
If it is less, their should be a Box 12 A entry of the shortage.
Also, double-check the amounts you posted in Box 5 and 6.
Box 6, Medicare taxes, should be .0145 (1.45%) of Box 5.
If it is less, their should be a Box 12 B entry of the shortage.

This often happens with tipped workers, whose tips get reported but not paid via their paycheck, thus resulting in not enough income to withhold the proper amount for Social Security & Medicare.