Google reCaptcha is required if you enable reCaptcha from the Admin Panel. It is used to prevent spam accounts.
Sign In: Sign in to the Google reCaptcha Admin Panel.
Register a New Site: In the top right corner, click on the plus + icon.
Choose reCaptcha Version: Fill in the required information and choose reCaptcha v2. Once selected, choose "I'm not a robot" Checkbox.
Add Domain: Add your domain name, then click "Submit".
Get the Keys: Grab and copy the SITE KEY and the SITE SECRET.
Admin Panel Configuration: Go to the following options and edit them:
Admin Panel -> Settings -> General Configuration -> Login & Registration
Edit the Settings:
Enable reCaptcha: Enable reCaptcha by clicking on the toggle.
Recaptcha Key: Enter the site key you created in the previous step.
Recaptcha Secret Key: Enter the secret site key you created in the previous step.