how to enable wallet in a confirmed node I installed/compiled my Bitcoin Core RPC client version v0.16.2.0 and i used --disable-wallet flag. 

I think this is the reason I am seeing "Method not found" errors when trying to GetNewAddress

how can i install/enable it after having fully confirmed node with up to date blockchain? 

os: Ubuntu 16.04.5 

solution 

accepted the answer below but in my case since i set it up sing apt-install bitcoind the wallet feature was already there, i just needed to flip the disablewallet=1 flag in my bitcoin.config
Yes, that is a wallet feature. You would have to recompile without the --disable-wallet flag, starting back at executing the autotools configure script: ./configure. See https://github.com/bitcoin/bitcoin/blob/master/doc/build-unix.md#disable-wallet-mode.

You also have to make sure you don't set the -disablewallet flag when you run bitcoind or have it in your bitcoin.conf file.

If you need to recompile, you could still back up the data in the .bitcoin folder, which includes the blockchain and UTXO set and copy it back over when the installation is complete. That way you don't have to download the blockchain again.