Data Validation -> Allow List -> "The formula you entered is not valid" (Excel online for Dropbox)

Hi,

I am using Excel online, in the browser, the one offered by Dropbox for free.

I am trying to get a simple list of items to appear as a dropdown list.

I select Data -> Data Validation, and then select "List" in Allow, and then click Source input and select the cells I want.

When I click Apply, I get the error "The formula you entered is not valid".

Any ideas why? The values I want to have in the dropdown list are super simple, like "Supermarket, Laundry, Gas, Transportation"...

I have done this several times in Excel for Windows with no issues.

Thanks