By default, Electrum displays your amount in mBTC. Perhaps you re trying to enter the amount to send in BTC, which is 1000 times larger.
That's a good guess;
But if it's an amount higher than his available UTXOs, the error "
Not enough funds" will be shown in the 'Send' tab without the "
use a lower fee" note.
Refer to this reply in page1 to see the instance where the error described by OP will appear:
/index.php?topic=5477026.msg63305152#msg63305152