you need to change theme only temporarily to see if it's theme related or plugin related issue

If I change the theme my site wont work. I can only use ReHub theme. Cant I change it in Rank Math plugin or elsewhere?

please, activate default theme and check. if you have the same problem,
please, report to developers of site which you use to share links

I have Site Title monkeycheap but it still dont work.

go to general settings of wordpress and set your site name. Also install and configure SEO plugin, like Rank math

Hello. When I send somebody link on the discord it showing up my username which I have on WordPress (tushovsky)  on the top of the link. How can I change it? Thanks.