There might be various reasons why the people aren't able to see the Open Shifts on their portal. Please check the following steps to make sure that the staff members could see the Open Shifts and apply for them.
Make sure that the system default setting People Can Apply For Open Shifts is switched ON in the Scheduling Settings and under Shift Assignment. The same setting is also on each site's profile, please make sure that it is switched ON there as well.
2. Position, Certificate and Site match
If you have any of the following settings switched ON - Require Position Match, Require Certificate Match, Require Preferred Staff Match - under the System settings - People Settings - Staff Portal, then you will need to make sure that the persons have the corresponding positions or certificates attached to them in order for them to be able to see the Open Shifts that are created with those details. With the Preferred Staff Match setting, the staff members have to be have been added to the Preferred List for the location.
If you are using departments you will need to make sure that both the location and the staff members are in the same department. If you have added a location to a department and not added the persons into it as well then they won't be able to see any open shifts on that location.