I had to upgrade my PHP version recently because it was sunsetting. I also upgraded WordPress at the same time. Doing so, caused the Crayon Syntax Highlighter plugin to fail. I used the fix I found here.
https://crunchify.com/fix-for-crayon-syntax-highlighter-wordpress-plugin-and-php-7-3-fatal-error/

Update on this. Since I had to upgrade PHP I’ve switched to the Enlighter plug-in