Seems like the transaction has been confirmed while checking through the block explorer. Negative confirmations is a well known bug present in the electrum software. Resyncing/re-connecting to a newer server will probably solve your issue I guess.
I use electrum, im not using my own nodes. why it not sync?
Electrum is a light wallet and it doesn't download the whole blockchain. Rather your wallet syncs to a server running a full node to broadcast transactions and update your balances. Servers can go down at times.
To restart/resync electrum do I need to reboot the server? Electrum daemon was running all the time. Shouldn't it sync?
Press the green button present on the bottom right of your client interface to resync with new server. When you click that, Press the server tab and select a new server from the list.
If that doesn't solve your issue and if it still persists you better need to delete the block headers.