[General] I have given a

[suniljain]

Look up success in a dictionary. Is it a verb or a noun?

I checked and found that "success" is noun. So we cannot use "success" after couldn't.

Can we use "he couldn't succeed" instead of "hadn't succeeded"?

 

[emsr2d2]

I checked and found that "success" is noun no full stop here so we cannot use "success" after "couldn't".

Can we use "he couldn't succeed" instead of "hadn't succeeded"?

No, that doesn't work. You can say "... he didn't succeed", "... he hadn't succeeded", "... he hadn't been successful" or "... he wasn't successful".