To set up your G Suite and to access Google Webmasters Tools (including Sitemaps), you need to verify your website with Google.
Step 1. Sign in to your Google Webmasters Tools account and go to https://www.google.com/webmasters/tools/home.
Step 2. Go to 'ADD A PROPERTY'
Step 3. Add your website URL.
Step 4. Verify your ownership.
Go to 'ALTERNATE METHODS' and copy the HTML tag.
Go to your Site Settings and go down to where it says 'CUSTOM <HEAD> TAG CODE'.
Paste in the Tag Code that was copied from Google.
Once you have copied and saved this code into your site settings, go back to the Google console and click on the 'VERIFY' button.
You have now verified your website with Google.