When this happens, go to https://matrix.itasoftware.com and find the specific itinerary you're looking for, which will give you the specific fare codes, etc. which you should use to book. I believe this ticket is available, it's just that the people you're speaking with can't find it in their system (that said, you don't say what fare you saw, so it's hard for me to be sure I see the same fare you do). Specifically, the bit under "Fare Construction".
Fare Construction (can be useful to travel agents)
CPH SK X/MAN VS NYC 500.77HLHF06MN NUC 500.77 END ROE 6.637696 XT 161ZO 167UB 39YC 47XY 27XA 124US 646YQ
I wish that Google Flights would offer a way to get this info in the UI; it's not common but not greatly unusual to be in this position (where Flights can find the fare but the UIs available to consumers can't book it).
Fare Construction (can be useful to travel agents) CPH SK X/MAN VS NYC 500.77HLHF06MN NUC 500.77 END ROE 6.637696 XT 161ZO 167UB 39YC 47XY 27XA 124US 646YQ
I wish that Google Flights would offer a way to get this info in the UI; it's not common but not greatly unusual to be in this position (where Flights can find the fare but the UIs available to consumers can't book it).