I did this and gave the same error
{"error Code": 5, "Error Description": "Unknown account"}
whats your account? can you see your account on block explorer ->
http://burst.cryptoport.io ?
my account
BURST-3N39-7T5J-CKPM-DLLBP
your account is not yet visible on network -->
http://burst.cryptoport.io/acc/BURST-3N39-7T5J-CKPM-DLLBPyou need atleast make one outgoing transaction to make your account visible to network, you buy alias, or send somewhere 1 burst