You can't check in multiple SharePoint Online items in a document library
Problem
You select multiple items in a SharePoint Online document library that are checked out, and then you try to click the Check In button. However, you discover that the control is disabled.
Solution/Workaround
To work around this behavior, use Check In for individual items.
More information
This is the expected behavior in SharePoint Online when a workflow is associated with the document library.
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