Problem with installing plugin in Audacity

I edit my songs in Audacity on my ubuntu 18.04 computer. Recently i downloaded GSNAP plugin(file extension : .so) and followed this article to install it. I should log in with root to edit the plug-ins folder. I entered sudo -i for logging in as root but I wasn't able to place the GVST plugin file into the PLUG-INS folder.

I also tried to do it completely in COMMAND LINE, but this was the result

Based on your comments, I believe I understand the issue. See, Linux is designed as a multi-user server operating system. That means it is intended to allow many sessions of different users to exist at once. When you signed in as root using by issuing the sudo -i command in a terminal, it didn't give your main user session any additional privileges. To Linux, when you opened the terminal window (most likely by opening the Terminal application), you created a new session of yourself. The sudo -i command then elevated that session to root privileges, but it didn't elevate the privileges of the existing GUI session. Thus, when you tried to move the file around in the GUI session, you didn't have the permissions to do so, since that was only granted to your terminal session.

The solution is fairly simple: either carry out the entire task in the terminal, or use a different target location. I would recommend the latter. Thus, instead of trying to move the plug-in into /usr/lib/vst/ (which isn't a terribly good idea anyways: dpkg is designed to have exclusive control over that folder), copy it into ~/.vst. In other words, create a new directory in your home folder called .vst, and copy the plugin file into that. This doesn't require root permissions, won't anger dpkg and apt, and can be done from the GUI.