I would like to answer a question but cannot

If it is grayed out, the post likely has this banner or something similar indicating it is locked for inactivity:

Spend your desire to help in newer threads. In the two decades I've been here, I've seen very few times where a poster thought to check for answers months after posting.

Al Babusis wrote:

I would like to answer a question but cannot. The option is not available.

Is there no "Reply" button below the post?

It appears you answered my question.

The thread in which you are not able to reply is likely an old thread that has been locked by the community moderators.