Functional Isomer of Formic Acid is?
Note the image of acetic acid (top) and the image of the carboxylic structural isomer methyl formate (middle).  As you can see the bonding location of the proton and the methyl group switch between the structural isomers.  Note the image of formic acid (bottom).  If you switch a hydrogen and a hydrogen you still get formic acid.  Yes, structural isomers of carboxylic acids make esters, but formic acid does not have a structural isomer, so the rule does not apply.

 Acetic Acid
 Methyl Formate
 Formic Acid