Hello,
Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
I suspect that you might have enabled the noindex option for your post tags taxonomy. To check this, please navigate to Rank Math > Titles & Meta > Tags and uncheck the tags robots meta if it is set to noindex as shown below:

You should set this option to default.
Looking forward to helping you. Thank you.
You are right Michael. I think there should be a hint. Those 2 options are conflicting.
Hi Zhao,
We appreciate your feedback.
So glad that the issue is resolved.
Hey, if it isn’t too much to ask for – would you mind leaving us a review here?
https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post
It only takes a couple of minutes but helps us tremendously.
It would mean so much to us.
Thank you.