I am having this problem of "Alternate Page with Proper Canonical tag" on my website. Also there is another error in coverage "Duplicate without user-selected canonical". How to fix these?
If you post your actual loaded canonical tag I can help further or confirm any issues. This is a more complex issue with many possible issues.
But I just fixed a critical canonical issue with my own site and thought I'd post it for others.
I had been making my home page cannon URL like so:
<link rel="canonical" href="https://aspectratia.com">
A static canonical URL like above was causing any URL with parameters to be disregarded AKA indexed as non-canonical.
https://aspectratia.com/?resx=1920&resy=1080&ratx=16&raty=9&page=1 = "Alternate page with proper canonical tag"
The fix was updating the canonical tag to include the params.
Because I want the URL with parameters to be considered unique pages since they define what unique content is shown.
<link rel="canonical" https://aspectratia.com/?resx=1920&resy=1080&ratx=16&raty=9&page=1">