Definition from Wiktionary, the free dictionary
|(negative prefix); not; no||
on; up; on top; upon; first (of two parts); previous or last (week, etc.); upper; higher; above; previous; to climb; to go into; above; to go up
|simp. and trad.
When placed behind a verb, 不上 has a meaning of "unable to achieve" + verb. The opposite to 不上 would be 得上 ("able to" + verb).