Do you understand what it says?

I found this sign at Otsuki station.

I don't know where you could buy a ticket for Fujikyuko-line other than Fujikyuko-line stations.

********************************************************************************
After I wrote this, I learned how to buy Fujikyuko-line ticket outside Fujikyuko area. You can buy one at JR Ticket Offices (midori-no-madoguchi みどりの窓口), JR Travel Service Center (View Plaza びゅうプラザ), or from the reserved-ticket vending machines (not from the regular green ones) from one month before your ride. JR-EAST

However, be careful if you transfer from JR to Fujikyuko-line at Otsuki station.
the station yard map
Platform #1 & #2 are for Fujikyuko-line. #3, #4 & #5 are for JR. When you get off at Otsuki, cross the bridge from #4 & #5, then turn right and down to the Fujikyuko-line ticket gate. If you don't have Fujikyuko-line ticket, go through the left gate and buy one from a vending machine. Fujikyuko-line allows neither Suica nor PASMO. Mostly there are only few minutes to transfer, but stay calm and don't panic!
Note from Fujikyuko (Japanese)