Hi!
I have notice that the header, the one that you put at the top where you name the page IS the H1 in YOAST. Therefore when you in the body create a H1 title it will say that you have more then one H1.
Try to start with H2 or H3 in the body or just a text paragraph and you will see that it will stop saying that you have more than one H1.
Good Luck! Hope you all solve this problem now =)
BR
Carla