You can tag photo on instagram but, some are steps of tag photo like. First of all you are enter your username# Second you are select subject of the picture. Third you are select the location. Fourth you are add comment about yourself. But there are difficult tag photo in instagram . read more
You can always tag the person you're trying to tag in the photo in the caption later, too. Just because Instagram won't pull up their username to tag doesn't mean that you can't get the users attention by tagging them in your caption. read more
You could be unable to tag people on Instagram due to a variety of reasons. The person you are trying to tag might have a private profile and have their settings set on people not being able to tag them. This is not uncommon and could be an answer to your question. Another reason could be that you are not following the person you are trying to tag. read more