Can anyone help what causes Gsuite to do this?
It says: GSuite error 550 5.7.26 mail is unauthenticated. The sender must authenticate with at least one of SPF or DKIM.
Tried to click the google support link but nothing helped at all. Can someone elaborate what causes GSuite to do this?
The bounce back message you're getting normally means that you do not have email authentication records properly configured in your domain. From the screenshot, your domain's SPF record does not allow emails sent from Google's IP address and since your email did not pass DKIM checks, you most likely do not have a DKIM record configured.
I suggest adding a DKIM record generated from Google Workspace's Admin Console and replacing your domain's SPF record with Google's. You can add both the DKIM and SPF records through your domain or hosting provider. You can get in touch with your domain or hosting provider for further assistance with adding the records