Everything breaks when pushing my local dev site with upgraded PopUp Pro Plugin to Staging/Live site

I upgraded to PopUp Pro on my local development site to test it before deploying live. Got the plugin working how I wanted and when I pushed to our staging site, nothing works! WMPU Dev dashboard says I have not installed WMPU Dev Dashboard, and that I have Version 4.4.5.4 which is the old WordPress Pop Up Plugin, before your major update to PopUp Pro. However, I have PopUp pro and am able to use it in the dashboard, but, the popups that I make don't work. Here are the warnings I get:

Warning: session_start(): open(/var/lib/php/session/sess_t2v4ntn0o25qsvquiat58mhod0, O_RDWR) failed: No such file or directory (2) in /var/www/staging.modernnotion.com/wp-content/plugins/popover/inc/external/wpmu-lib/functions-wpmulib.php on line 69

At bottom of page:

WarningUnknown0

WarningUnknown0
: session_start(): open(/var/lib/php/session/sess_t2v4ntn0o25qsvquiat58mhod0, O_RDWR) failed: No such file or directory (2) in on line : Unknown: open(/var/lib/php/session/sess_t2v4ntn0o25qsvquiat58mhod0, O_RDWR) failed: No such file or directory (2) in on line : Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/session) in on line

I am also unable to