If you have spreadsheets that include links and external references between spreadsheets, these cannot be updated using Excel online. This is a limitation from Microsoft Office.
You need to Co-edit the relevant spreadsheets, open each of the linked spreadsheets in FYI with the Co-edit button to open them in Excel Online. Then for each of the spreadsheets, click Open in Desktop App at the top of the Excel Online screen.
Refer to Using Co-edit for Excel Spreadsheets with Linked Cells in Co-editing an Internal Document.