Plugin shows “There has been a critical error on this website”

On attempting to edit the Pricing Table, screen displays: "There has been a critical error on this website. Please check your site admin email inbox for instructions. If you continue to have problems, please try the support forums."

Table displays perfectly in normal viewing mode of the web page.  Unfortunately, I am having to try to maintain this site after somebody else created it(!) so any help you can suggest would be much appreciated. This plugin has worked perfectly for several years (thank you!).