Let me share some information on how you can maximize the use of QuickBooks Self-Employed in tracking your transactions.
QuickBooks Self-Employed is designed to track your business-related transactions from self-employment. Currently, the ability to enter a purchase order is not available. What you can do is track your actual income and expense transaction in the system and we will account for all business-related transactions for your estimated tax calculation.
Did the information I shared above answer your question as to why you're not able to enter a purchase order in QuickBooks Self-Employed? I'll be sure to get back to you if you need more clarification about tracking your income and expenses.