"Please insert your Ledger. Verify the cable is connected and that no other application is using it.
Try to connect again?"
The thing is, the ledger is already connected.
That's most likely the problem... if the Ledger has already been connected to another app, to use it in another app you will most likely need to disconnect it from the computer, then reconnect it and unlock it again with your PIN.
Also, make sure that no other wallets or applications (like the Ledger chrome apps) are open as they may monopolise the connection to the Ledger device.
I'd suggest trying something like the following:
1. Disconnect the Ledger device
2. Open Electrum, and make sure that ONLY the wallet you want is open (Electrum can have multiple wallets open at the same time).
3. It will most likely show the Ledger icon in the bottom right corner with a red dot.
4. Click the icon, it should prompt you to connect the device (If it doesn't, close Electrum and restart it, it should open your Ledger wallet by default)
5. Connect your Ledger device, unlock it and select the Bitcoin App.
6. Click the "reconnect" button.