By default a Google Gsuite account disable the IMAP access, to access emails from a third party software you need to enable IMAP. So please go in your Gmail settings (generally on top right of your screen on Gmail webpage):

1. Go to your Google Admin console 

You need to log with your administrator account.

2. Go to section Apps and then GSuite and then Gmail and then End User Access.

3. In the Organizations section, click POP and IMAP access.

4. Enable POP and IMAP access for all users box.

If you check the box, POP and IMAP are fully disabled for all users in your organization.

5. Click to save changes.

Read official faq from Google: