In case you have neither U2F device nor mobile authenticator (phone) you can use backup (recovery) codes and enter a one-time secret reserve key wich you have downloaded while enabling 2FA for Confluence: U2F & TOTP process.
Step 1. Depending on which second factor (1 -TOTP) or (2 -U2F) do you use find Enter a two-factor reserve key link and press it.
Step 2. Find your Backup codes and enter a one-time password reserve key.