I have noticed this problem on and off for a few years.
This generally happens when you are working in the InstallScript view of a project. I think it usually happens in Basic MSI projects.
Anyway, you are working away and all of a sudden, you can no longer paste info into the script. No matter what you do, you can’t paste.
The solution is easy. Just save your project, close the project, exit out of InstallShield, then run it again, reopen your project, and go back to the InstallScript view. You will then be able to copy and paste again.
A simple solution to a problem you may encounter.