How do you save an environmental variable?
‘export’ doesn't write anything to file. It's for making variables available to subprocesses.

If you've never written anything to .bash_profile, it's unsurprising that it doesn't exist, since it's not created for you. You'll need to create it yourself and write that line within.

For example, touch ~/.bash_profile (to create the file) then edit it in an editor of your choice such as TextEdit (open using Finder or open -e ~/.bash_profile). Alternatively, nano ~/.bash_profile. Paste in export PROJDIR=/Users/$USER/Projects and save the file, then reopen your shell or source the profile.