Plugin Not Working

Hi Team,
When I installed and setup this plugin on my local server it worked really smooth, but when I try to install this on a server then things are not the same.

https://www.matalan.me/past-mailers/

If you see, I have tried 2 options which are not working for your reference on the above link.
Can you please assist us in knowing what the issue could be.

THanks in advance and looking forward to hear from you soon.

The page I need help with: [log in to see the link]