To spend a Bitcoin, you need a private key. What you did was to add an address which would not work. Unless necessary, I would advice you to avoid transferring your private key. If you absolutely have to,
1. Go to HELP>debug window> console on Bitcoin Core.
2. Use: walletpassphrase [insert your password here] 600 if you have a password.
3. Use dumpprivkey [ insert address] to export the private key.
Don't include [ ].
Next, go to electrum, go to Wallet, private keys and click either
1. Import - This adds your entire address into the wallet and you have to keep a separate backup of the private keys.
2. Sweep- This sends the entire balance to the wallet's address and you only need to backup the seed that you gotten during the creation.