How do I add text into an existing photo text box on MacBook Pro Photos app?

Are you talking about using Markup in the Edit Mode of the Mac Photos app? When I use Markup, I hit the text box and the word Text appears. I double click on it and type what I want instead.

Can you give us some detail in what's not working for you--exactly what steps you're taking and what buttons you're clicking?

Here's Apple's instructions:

Use Markup to write or draw on a photo on Mac - Apple Support

edit some txt in an existing txt box

I'm not sure what you mean by "existing txt box." You can highlight text in a picture, because Photos recognizes text so you can copy it. This is part of the original picture, and it can't be edited. It may be possible to remove it with Clean Up.

Or, you can add text to a picture with Markup, which is what I was referring to earlier. I was all set to say that, once it's saved, it's not possible to change the Markup stuff. You have to Revert to Original.

Which "Text" do you mean?

Actually, I just tried Markup, and I can edit text that I recently added on a picture. I could, in Markup, double click on text and change it. This may be new, or I may be crazy, or maybe my memory is just gone. I'm not sure.

I opened a Library that I had saved a month ago, and for pictures in that Library, what I said at the beginning was true-- I couldn't edit the markup, so I had to Revert to Original to remove the Markup stuff.. So it may be that only Markups from the most recent versions are editable.

Hmmm

Did you add the existing text/box originally in the Markup feature of Photos?

If not then you won't be able to change it.

Did you export the photo and then reimport it? That will cause the text box on that version to be sealed, i.e. uneditable.

If you did add the text yourself in your Photos library you must enter the Markup feature before you'll be able to select and add to the text box.

I can modify a text box I added with Mark-Up, as long as this has been the last edit I added.

But if I add further adjustments to a photo with a textfield, the textfield is a part of the edited version and can no longer be modified. Then I have to revert the photo to the original and start over.

Have you added other adjustments to your photo after adding the text? Or is the textfield a part of the original image? If the text is a part of the original you would have to remove ethe text with the clean-up tool and then add a new textfield with Mark-Up.