Hello everyone. I am monitoring some websites and a lot of these websites using Sectigo SSL Certificates. Heartbeat showing these websites with Sectigo certificates as x509: certificate signed by unknown authority. Why is this happening and how can I fix it. I am using debian machine. Thanks for your answers!
Welcome to our community! 
It's probable that that certificate is not in your OSs store. What OS are you on?
I am using Debian
I can't promise this will work, but maybe look at something like Adding trusted root certificates to the server
I tried this method and it didnt help. So I checked the certificates, I monitor like almost 200 websites and most of them (60%) have Sectigo certificates working just fine with no errors, but like 30 websites with the same Sectigo certificates showing me x509: certificate signed by unknown authority. Really dont understand why
So I downloaded bunch of certificates from here and Adding certificates to host machine added them by this method and restarted machine, and that worked.