If you don't have the option to approve a learner's SLO, it is likely that the SLO has not yet submitted their SLO for approval. You can view an SLO's status by clicking on the activity with the SLO step. A blank status indicates that the learner has not yet submitted their SLOs for approval. (More information on SLO status can be found here.)
To resolve this, the learner should navigate to the SLOs, scroll to the bottom of the page and click Set Objectives & Request Approval.
Once the SLOs have been submitted by the learner, you can approve and rate them as usual.