Hello and welcome,
There is a similar topic about this already, this one: Cannot renew free Basic license on 5.2.2 — register.elastic.co reCAPTCHA shows "Invalid site key"
There is no updated from Elastic yet, but considering that version 6.X reached EOL in 2022 you should look into upgrading to version 8.19 or 9.4 as soon as possible.
There is no guarantee that this would be fixed or that this site will exist for much longer.