Can two acute angles form a linear pair ? If not why?

No, because they wouldn't add up to 180 degrees.
An acute and obtuse COULD.                    

Because acute angle means it shall be less then 90 degrees and more then 0 degrees. so if we take the biggest acute angle and it with itself  it will be less then 180 degrees. Eg:-
89 + 89 =   178

Because acute angle means it shall be less then 90 degrees and more then 0 degrees. so if we take the biggest acute angle and it with itself  it will be less then 180 degrees. Eg:-
89 + 89 =   178

no sum of 2 acute angles it cannot be 180 

no, two acute angles cannot form a linear pair as acute angles are angles which measure less than 90°.
let's consider two acute angles, 60° & 80°.

so, 60°+80° should be equal to 180°
but 60°+80°=140°>hence, proved

60° + 80 ° = 140 ° > hence, proved

however the requirement at least 1 angle should be 90 degree in a linear pair or supplementary angle