I am trying to use blood oxygen app but says that I can not use it cause I am under 18 ( I am 50)

Hello, as it says at the topic, just bought an Apple watch series 6 GPS and cellular under VOdafone.

Trying to use the oxygen app, says I can not use it cause I am under 18.

I wish I were but I am not and can not see how to sort this problem out.

Any help will be appreciated