echo $DISPLAY returns nothing

[mahesh]

This is a rabbit hole I fell into because I wanted to script a window switch in tmux.

I saw that the Ubuntu project had documentation on something called "xdotools." I gave it a try, but it always returned this:

Error: can't open display: (null)
Failed creating new xdo instance

So I found a few posts on stackexchange and one of the steps was to run "echo $DISPLAY" to find out what display to specify for xdotools, but it returned nothing.

I thought it was an issue with SSH but after troubleshooting for a while I plugged directly into the server and that didn't change anything.

I don't know where to go from here.
Thank you in advance!