The difference is subtle.
Won't - definite and certain.
Wouldn't - hypothetical.
Both are correct, but have slightly different meanings:
"If the meeting runs late, I won't be able to make it." (I definitely won't be able to make it if the meeting goes late.)
"If the meeting runs late, I wouldn't be able to make it." (Hypothetically the meeting could go late, and in that case I wouldn't be able to make it.)