Sadly, you can't import a SegWit private key in WIF format (starting with "K" or "L") into Electrum... as the "Import Bitcoin Address or Private Key" wallet creation option only generates "Legacy" wallets and will generate the "Legacy" address that starts with a "1".
I believe this is why the "Sweep" option is also returning "No inputs"... it is only checking the "Legacy" address generated from your WIF key
You'll need to either manually craft a transaction and broadcast it... or find another wallet that allows importing of SegWit private keys... or wait for Bitcoin Core to sync
EDIT: you could also try the "dumpwallet" command from Bitcoin Core and see if it gives you a "
ypriv" Master Private Key in the output file... if it gives an "
xpriv" Master Private Key, Electrum will still treat it as a "Legacy" wallet