Dear Fernando,
Thanks for your prompt reply,yes,it is the weekly tabs which I can not choose,please see the picture attached.
as to the possible reasons you mentioned above,I set up a new course as a default FN format course,I did not change anything here.And the start date is in sept.30,and today it is Oct.1st.it should not be the start date issue.
And I have to different users,one is the creator of the course and the other assigned as user only.When I logged in the student user(student role only ),the tabs does not work for me.they are in grey shape,and as I notice that only the blue tabs can be selected.