Page wont let me set up a 2-factor authentication. Help!

[Londongirl2020]

Hi

I click to '2-factor authentification', press 'text me a code' and click 'set up'

It asks for my number, which I fill in.

I click 'Next'

It then says 'The mobile number you've entered is already registered.'

But it clearly isn't set up, because it doesn't send me a text message when I log in.

Any ideas?

Thanks