SharePoint Online lists in Office 365 – 5,000 item threshold
“The number of items in this list exceeds the list view threshold, which is 5000 items.”
To enable metadata-based navigation, follow the Enable Metadata Navigation and Filtering, and Configure Metadata Navigation sections at the following Microsoft website:
After metadata navigation is enabled, the list contents will become available. We recommended that you then reduce the list contents to less than 5,000 items.