If you are unable to upgrade your Mac to macOS high Sierra due to error " The Recovery Server Could Not Be Contacted" . First check if the system time is correct and then make sure your Wi-Fi is secure and also reliable. If your Wi-Fi disconnect intermittently then it may drop the connection with Apple server while downloading the contents.
Also if possible try to use the Ethernet connection as that could make your internet connection reliable.

As per iguruservices the error Recovery Server could not be contacted occurs due to several reason and the solution to which is listed at The Recovery Server Could Not Be Contacted - https://iguruservices.com/support/recovery-server-could-not-be-contacted-error/ . They have explained the reason and solution so nicely.

This becomes a common error almost every time apple releases a new version of macOS. 70% of the times this error happens due to overwhelming download requests to Apple server. When Apple releases a new version of macOS, a lot of users try to download the new macOS version at the same time. Apple servers become so busy processing the download request by them. So try to download again after an hour and see if that helps. But this time Apple implemented a new software delivery method for macOS High Sierra installer file.