The online version of SharePoint in Office 365 does not accept email attachments the way the on-prem version does. There is a way around it. Let me show you in this brief post.
First create a new shared mailbox in the Exchange Admin center (Admin >Exchange Admin > Add). Return to the landing (home) page.
Then create a new Team site or use an existing team site (upper left corner).
Open Power Automate and go to Templates. Find the template that says “Create a File in SharePoint When New Email Arrives in the Shared Mailbox.”
Fill in the configuration items and set the URL to your Team site. (I prefer to delete the “Send Me a Mobile Notification” section).
Now, merely test it by send an email with an attachment to the shared mailbox you just created. (It might take 15 minutes or so to show up).
Voila! The document should show up in the designated library.