Should there be a fault message with a newly installed combi boiler?

Hi F28 is usually a low pressure fault code on vaillants, meaning there is insufficient water in the heating side of the boiler. This shouldn't be happening on a brand new boiler, suggesting perhaps a leak somewhere? You need to get the original installer back to fix this. You could try refilling the boiler using the filling loop, your installer should have showed you how to do this when handing over the boiler to you.