Location is an important field as values on the Inventory Detail form is based from the Item and the Location. Without Location, NetSuite cannot determine which valid Inventory Numbers to show.
To resolve, make sure that the standard Location field is exposed either to at least on Main Line or as a Column on the line level.
1. Navigate to Customization > Forms > Transaction Forms.
2. Click Customize before the name of your Custom Form.
3. Either one of the steps below can be done or both.
a. On Screen Fields > Main tab > mark the Show box for Location.
b. On Screen Fields > Columns tab, mark the Show box for Location.
4. Click Save.