Are you sure that your blockchain.info account have bitcoins in it and the addresses which hold them is not watch only? If you are trying to transfer a watch only address then it is not possible as you don't hold private key of that address.
Yes, I'm sure that I have bitcoins in that blockchain-account and it's not watch only, because I have already received and sent money from there.
But I thanks for putting me in the right direction: The address where I got the BTCs in my Blockchain-Account is an "imported address", as I got my BTC in another wallet before. Could that be part of the problem?
UPDATE: Thats it, thanks! I transferred the BTC form the "imported address" into my wallet. And there they are in Electron. Great! Can I delete my Blockchain-Wallet no without loosing the BTC in my Electrum-Wallet?