Do I need to amend 2020 1040 if I listed FWT on the wrong line. I ent'd 1,200 on line 25b (form1099) instead of line 25a.

How did you get it on line 25b?  Didn't you enter a W2?  You had exactly 1200 withholding?  What was it from?  I would wait for a letter from the IRS.  They might send you a refund since it wasn't on 25a then later send you a bill for claiming too much on 25b.  

Did you file e-File or on paper?

Did you have a w-2 and 1099 ?
If IRS does its job when processing your return it should move amount on line 25b to 25a.

You shouldn't have to do anything.

Actually, it's line 25d that counts.