Most probably this error occurs since you are using SharePoint Server 2013 Client Components SDK or SharePoint Online Client Components SDK against SharePoint 2010 .
Web class does not expose RegionalSettings
property in SharePoint 2010 CSOM.
To summarize, it does not seem possible to retrieve Regional Settings using SharePoint 2010 CSOM since Web class does not expose RegionalSettings
property.
How to determine CSOM SDK assembly version?
Assembly assembly = Assembly.GetAssembly(typeof(ClientContext));
FileVersionInfo fvi = FileVersionInfo.GetVersionInfo(assembly.Location);
string version = fvi.FileVersion;
How to determine SharePoint version via CSOM?
using (var ctx = new ClientContext(webUrl))
{
ctx.ExecuteQuery();
var version = ctx.ServerLibraryVersion;
}