The limits functionality that drives purchase requisition approval is dependent on the use of Jobs. This means businesses who want to have Approval limits or Spend limits directly related to the Employee (Worker) or User Accounts are not able to do so, especially in an environment where D365 HR is also being used and the Jobs functionality is being used to drive Position management.
Work Around (Where D365 Finance and Operations HR is not being used)
The first option work around, in a scenario where the HR functionality is not being used, or specifically where the Jobs functionality is not being used, is to create a Job for each User or Employee. This may need a little additional development which automates the creation of a Job for every user or employee(worker) record created. By doing this, you open a door where limits are maintained directly by employee or user.