Yes, you can undo a rejection for an Onboarding Request or an Approval.
How to undo an Onboarding request:
Go in the "Onboarding" tab on the navigation bar and filter for all "rejected" requests.
As you can see, for all requests which have been rejected there is a "Retrieve" button on the right side. Click on "Retrieve"
and confirm your decision. Now the request status will be switched from "Rejected" to "Pending" and you can now approve it.
How to undo an Approval:
Go in the "Approval" tab on the navigation bar and filter for all "Rejected" requests.
Once filtered, you'll have to click on the action buttons on the right side. You'll have the option "Retrieve" displayed. Select it and confirm your selection. Now the Approval status will be switched back from "Rejected" to "Pending". Now you can approve the request again.