Hello, I’m sorry to hear that you’re experiencing issues with the canonical tag not showing up in the Chrome Dev Tools.
To troubleshoot this issue, please ensure that you have enabled the Advanced Mode in the Rank Math plugin. You can do this by navigating to WordPress Dashboard > Rank Math > Dashboard and enabling the advanced mode.
Once you have enabled the Advanced Mode, you should be able to see and update the canonical URL in the Rank Math meta box, as shown in the screenshot below:

Under the Advanced tab, in the canonical URL field, simply enter the URL that you want Google to index.
If you’ve already done this and are still experiencing issues, please ensure that you have saved your changes after updating the canonical URL. Once you’ve saved your changes, Google should re-index the page with the new canonical URL.
If you continue to experience issues after following these steps, please feel free to reach out to us by opening a support ticket at https://rankmath.com/wp-content/uploads/2023/01/advanced-mode-8-scaled.jpg Our support team will be able to provide you with more specific guidance based on your website’s configuration.
Thank you for bringing this to our attention, and we appreciate your patience as we work to resolve this issue for you.
Sources:
Alternate Page with Proper Canonical Tag Status in Google Search Console » Rank Math
How to Fix Duplicate Meta Tags » Rank Math
How to Fix “Duplicate, Google Chose Different Canonical than User” » Rank Math