E-mail can only be set up for domains that are bought in Site.pro or delegated.
Go to Google Workspace;
Press "Get Started";
Follow the steps and create your email account with your domain;
Input payment information to finish the setup.
Sign in to your email account;
Follow instructions (received in email) to finish setup;
Login to your Site.pro account and next to your domain click "Cog" button - Mail server;
Choose "Gmail (G Suite)" and press save;
Please be informed that google has no free plans.