Добавлен GET с POST, теперь код работает
@app.route("/",methods=['GET','POST'])
def free():
if not google.authorized:
return redirect(url_for("google.login"))
event = {"timeMin": "2019-03-31T00:00:00Z",
"timeMax": "2019-04-01T00:00:00Z",
"timeZone": " Asia/Calcutta",
"groupExpansionMax": 3,
"calendarExpansionMax": 1,
"items": [
{
"id": "abcd@gmail.com"
}
]
}