To install a Monero core node on Ubuntu and enable JSON-RPC, you can follow the steps below:

1. Update your system:

   sudo apt update
   sudo apt upgrade

2. Install the required dependencies:

   sudo apt install build-essential cmake pkg-config libboost-all-dev libssl-dev libzmq3-dev

3. Clone the Monero repository from GitHub:

   git clone --recursive https://github.com/monero-project/monero.git

4. Change to the Monero directory:

   cd monero

5. Build Monero:

   make

6. Install Monero:

   sudo make install

7. Start the Monero daemon to synchronize with the network:

   monerod

This will begin the initial synchronization. Wait until the synchronization is complete before proceeding.

8. Stop the Monero daemon by pressing Ctrl+C.
9. Create a configuration file for the Monero daemon:

   nano ~/.monero/monerod.conf

In the configuration file, add the following lines to enable JSON-RPC:

   rpc-bind-ip=0.0.0.0
   rpc-bind-port=18081
   restricted-rpc=true

You can customize the rpc-bind-port value if necessary.

10. Save and exit the configuration file by pressing Ctrl+X, followed by Y and Enter.
11. Start the Monero daemon again, this time enabling the JSON-RPC:

   monerod --rpc-bind-ip=0.0.0.0 --rpc-bind-port=18081 --restricted-rpc

The JSON-RPC interface will now be accessible on the specified IP address and port.

You have successfully installed a Monero core node on Ubuntu and enabled JSON-RPC. Make sure to secure your node and follow best practices for running a cryptocurrency node.