That check only checks one page, not the whole site.
Running the check on the homepage shows no h1 there.
Running that check on an article gives this result.
Header Tag Extractor
H1 1
H2 2
H3 7
H4 15
H5 0
H6 0
As I said, your homepage has no h1, but the articles do.
You should add a h1 to the homepage.