If the blockchain is not synced, how are you going to see any confirmations? You don't have data for the current network status yet.
I don't think that's how it works. You should be able to send coins if the coins you are sending are spendable (enough confirmations).
I have confirmed something is wrong because the node is now fully synced and the transaction still has a "?" icon and has the "Status: 0/unconfirmed, in memory pool" message when double clicked. Nothing happened when looking at the explorer too.
The fee was the recommend one by the wallet.
I could try -zapwallettxes (which is annoying since it takes a long time) and try sending it again but I don't get what went wrong.