google adwords Value is required error

When adding a new Google Adwords campaign I was getting the error

'Value is required'

However Google was not telling me where the error was.

I found the problem was in the 'Ads Extensions' area. where i'd selected 'Product' but not choose my Merchant account from the dropdown. Once I'd done that the problem was solved.

I'd expect the same error may occur if you'd choose 'sitelinks', 'location' or 'phone number' from the list.

Hope this helps you ;)