Critical error - can't post comments

I've been building a website (not a professional and don't know much about coding). Everything looks good, except for I can't post comments on my blog posts. I get a critical error message. Please see below and please advise. I'm using the Blocksy theme and Gutenberg builder. I also recently migrated my website from one hosting provider to another, not sure if that's relevant, but I'm working in a staging environment and it doesn't work there either. I've removed the second level of my domain because my website is not public yet. Many thanks in advance.