If you cannot approve a Purchase Order as it is saying you have not completed a required Custom Data Field, you will need to edit that field to make it not required.  

Having a Custom Data Field set as a required field means it will be required on all Purchase Orders before they can be approved. 

Follow the below steps to edit the Custom Data Field to make it not required. 

1. Go to the settings cog on the top-right of the page and select Company Settings from the list 

2. From the list on the left-hand side of the page, select Purchase Order Creation and then select Custom Data Fields from the sub-list 

3. Find the Custom Data Field you need to change and click the edit icon to the right of it 

4. Switch off the button for ‘Make this a required field?’ and click ‘SAVE CHANGES’