A user cannot create new or refresh a current Sandbox Vault on the Sandbox Vaults page in the connected Production Vault.
Customers are limited to three Sandbox Vaults at any time. Refresh and creation is unavailable if, for historical reasons, the Production Vault has more than three Sandboxes connected.
Remove all but three of the Sandbox Vaults to enable the refresh option
Remove more to create new Sandbox Vaults according to the three Vault limitation.
Vault Help Documentation: Vault Help - Administering Sandbox Vaults: Creating Sandbox Vaults
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