After the Canadian Grand Prix was cancelled F1 announced that the Turkish Grand Prix would replace it. But with new travel restrictions and the UK having placed Turkey on the "red list" for travelling it is unable for a majority of the teams to fly there.
In replace of the Turkish Grand Prix will be a second Austrian race with the French Grand Prix moving up a week this creates a triple-header race weekend. The French Grand Prix will be on the 18th - 20th of June while the Austrian (Styrian) Grand Prix will be on the 25th - 27th June. The second Austrian race will take place on the 2nd - 4th July weekend.
These changes still allow 23 races on the 2021 calendar which is a record breaking season for Formula 1 beating 2012s record of 20 races.