Edit 2:

Looks like a SPS proposal passed that nixed the outbound transfer of Vouchers. Can only transfer to a player now. There's likely a Pool that they can be tossed in.

Edit 3:

They can be swapped for DEC
https://next.splinterlands.com/voucher-hub?tab=swap&view=all

VOUCHERS NO LONGER OUTGOING

Click the DEC icon at top of page
DEC Wallet appears
There's tabs near the top
Click VOUCHERS
This will bring you to a new window
Click the transfer icon
This will bring you to a new window
Its in this window that you can transfer your vouchers to Hive-Engine or player of your choice

Edit: Looks like the transfer to Hive-Engine option isn't available
I'll look further into it.