Auto Outline greyed out, and none of the usual fixes work

I have an Excel workbook checked out from our SharePoint site so I am the only person who can update this file. I want to have collapsed rows for easy viewing. I've read how to do this, using Data > Group > Auto Outline, however auto-outline is greyed out.

If I use Group, the header row collapses with the sub-rows. My header row is at the top, and because Auto Outline is greyed out, I can't change the default setting to recognise that.

Does anyone have any idea why this menu item is greyed out? Workbook is not protected, sheet is not protected, tabs aren't grouped, Advanced Options says "Show outline symbols if an outline is applied"...

As I said in my original post, this option is already checked...

My issue isn't that the symbols aren't showing... My issue is that Auto-Outline is greyed out, so therefore I can't adjust my header row from bottom to top, etc.

I have some Excel files stored in SharePoint where I can see Auto-Outline, and others where I can't, and there seems to be no reason why this is so... Due to the hundreds of comments on the forum about this issue, I'm not the only one wasting hours and hours of effort trying to do something that MS says I can do with a couple of clicks.

Can anybody else help? This has been sitting unresolved for a couple of weeks now...

Again MarvinP, as stated in my earlier post, it isn't a SharePoint issue since I have other spreadsheets stored on SharePoint libraries where this works as expected... This is now twice you haven't read my post properly and yet you reply anyway...

Hi Diana - I know they don't work. I don't know how to get them to work. You haven't been any help at all.

I'm so disappointed with the lack of response on this forum that I'm quitting it.