is there to way to add comments in crystal report at run time .
Else can we enter comments in text box at run time i.e for every line

please suggest

Enter your comments in a parameter field and this will then show up on your report.

Not sure if that is what your after? Or are you asking after the report has been run?

after the report has been run i want to enter comments

I dont think you can as the comments would need to be stored somewhere other than by either:
1. Exporting to excel ro similar and updating against each line
2. You update the corresponding records with comments in your database and add a field there for users.