Would anyone have any thoughts why my clipboard isn't working? It works in all other programs. I can copy text from other programs into Blender's Text Editor but I can't copy from within Blender? I have tried Copy / Paste and Ctrl C , Ctrl V but nothing works. Even tried reinstalling Blender. This is on Ubuntu 18.04 .
Make sure blender is in focus and your mouse is over the specific, blender internal text editor window.
Unlike any other text processing application blender not only needs to be in focus but also requires you to have your mouse over the specific (text editor) subwindow while copying text.
Just tested with ubuntu 18.04 and blender 2.79 from builder.blender.org
Does Blender have active focus before trying to copy? (Click in it's window somewhere.) Also keep in mind that even when Blender is the active software, each entry field the cursor goes over may be taking focus. Thus cursor position may be important in this. So after highlighting the text, keep the cursor over the text window pane when copying.
Then the same with switching back to other software like a text editor. You have to make sure it is the active software with focus before pasting.
I'm on Mint myself, but it shares much of the underlying OS software with Ubuntu.