I have limited website experience, but took over a site some years ago. The "Contact Form 7 (WordPress)" page has suddenly started to show "ERROR for site owner: Invalid site key", and cannot fix it

I have followed several YouTube videos including this one:

I have used the V2 version of reCAPTCHA.

I have removed all the other plugins I have tried when trying to fix this, generated new keys etc; input the [recaptcha] in the script as directed, and performed all other steps re integrating etc