Are there direct flights from varanasi to lucknow?

There are no direct flights between Varanasi and Lucknow.
You have to go to Delhi and come back, if you want to fly.
Considering time required for security checks at the airports

or even otherwise, train journey is faster and cheaper.
Varanasi Jn to Lucknow Nr - Varuna Exp (4227) by AC Chair Car (CC) is Rs 327.
Duration: 4hr 55min (9 stops).