Photo manually add people

When Photos first loads an image, it scans the picture to see if there's anything in there that it thinks might be a face. If it finds what it thinks is a face, it puts a little circle icon in the Info preview to let you know. Then, if it's unidentified, it lets you offer a person's name to go with the face.  If Photos doesn't find the sort of details to make it think that there is a face there, then the face icon doesn't show up, and you have no "face" to identify. The picture is never scanned again and, even if it were, Photos wouldn't see anything it didn't see the first time.

So you cannot add this picture to a Person album. On an iPhone or iPad, you can't directly name a face that Photos doesn't recognize.

One "workaround" would be to do it on a Mac, which allows you to select people and name them, even if the face isn't recognized. It's what I do, but I'm guessing you don't have a Mac, so this won't work for you.

Another thing that many of us do is include the name in the Caption field in the Info window. Then, if you search for, say "Caitlin Clark," you'll get the ones that Photos recognized and also the pictures with "Caitlin Clark" in the caption. 

This is good practice in any case. Face tags can be lost in OS updates, and Face IDs can't transfer with an exported image file. But captions do export with the picture. I include the name in the caption for every picture that has a recognized face, as well as for un-recognized faces.

Hope that helps...