I got the error "Invalid value for property" while setting background-color to a Hex representation of the color and while setting Font Size to a double number. I have observed this especially while working with older Xamarin projects.
I could resolve this issue by updating the Xamarin.Forms nuget package.
To update the nuget package :
Right click on the Solution and Go to "Manage Nuget Packages"(Or go to Project menu > Manage Nuget Packages).
Under "Update" tab, you will see a list of nuget packages for which latest versions are available and Xamarin.Forms nuget will be one of them.
As soon as I updated this nuget package, the "“Invalid Property Value” error disappeared.