@shreyak.garg We too are facing the same issue. Just to further shortlist the two differences between you and the user - in our case both the user and the WA have SSO enabled. So, essentially, everything else being equal - being WA seems to resolve this issue.
Please do update this thread in case you happen to find the cause/solution.
@shreyak.gargIf I'm understanding you correctly; you have a numbered list with selective access enabled. There are also orphans in the numbered list. The action which is causing errors is mapping parents to these orphans.
Anaplan correctly denies access to orphans in a list with selective access enabled to non-WSA users. This is why the action is causing the error.
I suggest either mapping the parent in the same action that is currently creating the orphans. Or alternatively, the orphans being created currently should be mapped to a temporary dummy parent. This will allow all users access to the orphans and the last action should successfully remap the 'orphan' to the desired parent