Verify that the access nodes for SharePoint Online meet the guidelines. Access nodes are virtual machines, cloud instances, or physical servers that run backups and other operations.
Note: For high availability (which is recommended for the first full backup), use 2 or more access nodes.
Service accounts or apps
5 or more
10 or more
Data backed up each day
CPU or vCPU for the access nodes
RAM for the access nodes
Note: Ensure that at least 100 GB of space is available on the access node.
To fetch the size and the number of items in a site collection, see Powershell Script to Fetch Site Level Statistics.