Mismatched canonical tag in HTML and HTTP header issue means that URLs that contain a double slash after the domain part (e.g. https://sitechecker.com/pricing//).
A double slash in the URL path is valid and will respond in the browser, but is typically unwelcome, as this could cause duplicate content issues if the CMS delivers the same content on two URLs (i.e. single slash and double slash).
Checking Mismatched canonical tag in HTML and HTTP header is important but not enough to rank good enough!
Check not only the issue but make a full audit to find out and fix your technical SEO.