Explanation
One possible reason why a manager receives an error when trying to approve a record is that the user in the next approver step is inactive or missing, which means there is no valid user to assign the record to after approval. To troubleshoot this issue, an administrator should check if the user in the next approver step is active and exists in Salesforce; if not, they should activate or create the user or change the approval process to assign the record to another user. Adding a delegated approver for the next approver in the process does not solve this issue because delegated approvers are only used when approvers are unavailable; they do not replace approvers who are inactive or missing. Updating the field level security to view on fields that are updated in the process does not solve this issue because field level security does not affect approval processes; it only affects what fields users can see or edit on page layouts. Reviewing the page layout to ensure fields updated in the process are visible does not solve this issue because page layouts do not affect approval processes; they only affect what fields users can see or edit on page layouts. References:
https://help.salesforce.com/s/articleView?id=sf.approvals_considerations.htm&type=5