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.
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.
You really should have just created a new wallet with a new seed. Creating a new address on bitaddress or any other website is a very bad idea.
Didn't you save the QR Codes or the private key as key? Just the screenshot? It's really hard to copy a full private key without any mistakes.
I'm almost certain you made a small mistake when copying it from the screenshot. It must be perfect, including the case. If you swap a single character or make it lowercase instead of uppercase then it will fail.
Just try again. A good method is to copy everything twice and then compare you copied it exactly the same both times.
If the QR Codes of the private key is included on the screenshot then it will be much easier to just sweep it by scanning that code using a mobile wallet, for example Coinomi (install it, create a new wallet, backup the seed, go to Bitcoin, options > Sweep and touch the QR Code icon).
NVM it was all my mistake. I couldn't figure if a part of the text was an "l" or "1". Sorry and thank you for the help. Best regards!
Good. Never do that again. Just use Electrum or other wallet and backup the seed.
