Please Find below screenshot for more clarifications

Able to view the Requisition at Offer Letter Stage ( In Sourcing).

Unable to view requisition at Offer Letter Acceptance stage (In Onboarding)

Reason for the same is :

In Talent Acquisition, number of positions which is raised and assigned to the recruiter is blank that means there is no positions assigned to the recruiters hence the requisition will be visible in sourcing and not in onboarding