The System cannot find the file specified error while deleting folder

At a command line run:

ECHO %UserProfile%

What does it return?

Open that directory in Windows Explorer and double check that a folder called AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\abc exists in it.

I'm thinking that maybe it doesn't exist, and the folder you're looking at that you said does exist, is in a directory with a root that is different from %UserProfile%.

Update

Open a new command window by doing the following, which ensures that it's in Administrator mode.

On a Shortcut to a command window, Right Click > Properties > Shortcut > Advanced and check the "Run As Administrator" checkbox, and click OK twice. Then run the command prompt via the shortcut. You'll know it worked if it looks like this, with the word "Administrator" in the title, see screenshot below. Then try running your RD command again.