Unfortunately, the route is temporally suspended. It may be normal operation within a couple of days, however it is still pending.
I would recommend that you take a highway bus from Shibuya instead of Shinjuku Expressway Bus Terminal as the highway bus from Shibuya to Kawaguchiko is normally operating at the moment.
In that case, you need to cancel your previous ticket first and then book a new ticket from Shibuya to Kawaguchiko.
By the way, if the bus you have reserved get suspended, you can get a refund for the ticket. Please note that you need to cancel your reservation from the "My Reservation" on the website within 30 days after the bus was suspended. Otherwise, you cannot refund your ticket.
I hope this is clear to you.
Have a nice trip in Japan!
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