Can you check the debug.log file because it seems your bitcoind is not running based on the error message you gave.
tail /home/ubuntu/.bitcoin/debug.log
The output of the debug.log could help a lot to see what's your problem. But there is a good chance the directory bitcoind is trying to access either doesn't exist or is doesn't have access right to read/write there. Or you are having errors in your confguration file. In the last case delete your bitcoin.conf file all together and try again.