That happens when the private key is not recognized as valid.
You can go to the option "Wallet Details" on bitaddress and double check it's OK, or try another format (the one that starts with 5).

If you keep having problems, you can try creating another address, a dummy one which you'll never user and you can share here, just so that we can confirm you're doing it right.

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But, why are you importing a private key there? If it does funds, you should sweep it instead (Wallet > Private key > Sweep), it's much safer.
If it's a fresh new address, then don't create it on a website! Just create a new wallet on Electrum with newly generated seed.