Hi pooya87, thank-you for your answer
are both wallets (online and offline) the same version? tell us which version is it.
Yes, both wallets (online and offline) are eguale version: 2.83. The OS is Lubuntu 16.04.
also is your wallet created from seed or did you import the private keys?
I exported from the wallet offline the
public keys, not private keys.
In the wallet online I've imported the public keys after. (As the guide
http://docs.electrum.org/en/latest/coldstorage.html)
make sure you are spending "coins" that belong to the private keys that the offline wallet has.
because the reason why that button is not enabled is either because electrum doesn't have the private keys or it recognize the unsigned tx format.
I believe yes, I think the procedure is correct
But something is wrong
Thank-you very much for your help.