How does Flush error occur and how do I solve it?

Solution

class Users(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(15), unique=True)
    email = db.Column(db.String(50), unique=True)
    password = db.Column(db.String)
    item = db.Column(db.String(25))
    amount = db.Column(db.Integer)

id = db.Column(db.Integer, primary_key=True, Sequence('user_seq'))