Add a text box to a pdf

I've been using Acrobat for 20 years. I  OFTEN add my own notes to a pdf. Now, I just tried to add text to a pdf by right-clicking and choosing Add Text. This results in all text boxes becoming active and I'm able to add text to THOSE EXISTING boxes, but i want to add my own text OUTSIDE of the existing text boxes. Been doing this for years. Suddenly I have a pdf that won't allow this. Is there a mysterious permissions issue here? This is a paid-for, downloaded pdf.  Actually, it's a knitting pattern. Acrobat is useless to me if I can't add my own text to pdf's. 

Using Acrobat 2023.008 20470 64 bit on Windows 11 Creative Cloud subscription (which is expensive for me!)