Having trouble transfering private keys from Bitcoin Core to Electrum, for want of not having to wait for sync
Redo the import. Start via file > new/restore, enter a unique filename and click next to begin the process. This time prepend one of the following to the private key:


p2wpkh: if the address in question begins with bc1
p2wpkh-p2sh: if the address in question begins with 3


For example:

p2wpkh-p2sh:Ky9mddDtaqcLC1vv6egmgQpiZh8Lgh111LnJSyJENxJs3oW93bjD

You can also click on the info button on the private key entry step to see further examples.