this command is not available unless the connection is created with admin-commands enabled

Updated answer for StackExchange.Redis:

var conn = ConnectionMultiplexer.Connect("localhost,allowAdmin=true");

Note also that the object created here should be created once per application and shared as a global singleton, per Marc:

Because the ConnectionMultiplexer does a lot, it is designed to be
shared and reused between callers. You should not create a
ConnectionMultiplexer per operation. It is fully thread-safe and ready
for this usage.