first sorry about my english

i'm study bitcoin

so develop my own altcoin basic bitcoin

in my virtualbox use ubuntuA, ubuntuB

my altcoin.conf
rpcuser = user
rpcpassword=pass
addnode=192.168.0.139
rpcallowip=192.168.0.0/16

netstat -anp |grep 8486
127.0.0.1:8486 -listen
0 ::1:8486 -listen

8486 is my port

and use cpuminer

./minerd -0 127.0.0.1:8486 -u user -p pass -a sha256d --no-longpoll --no-getwork --no-stratum --coinbase-addr =3E~~~~~ -t 4

and return

4 miner threads started using sha256d algorithm
binding thread 0 to cpu0
binding thread 1 to cpu1
binding thread 2 to cpu2
binding thread 3 to cpu3

HTTP request failed the requested url returned error : 500 internal server error
json_rpc_call failed retry after 30 seconds

ㅠㅠ i want make my first genesisblock.. and make my own altcoin.. please help me..