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 Co-editing Excel Spreadsheets with Linked Cells, Macros and other Controls, or Links to specific Worksheets.