Location Mandatory:
This is an option on your Inventory Setup. Go and uncheck the box if you don't use Locations. However, if you have Locations, we strongly encourage you to keep Location Mandatory turned on.

Dimension Requirement:
Look at the GL Account where the entries would land when Posted. Under Dimensions you will see a "Value Posting" field which likely has "Code Mandatory" set. This means you are not able to post a transaction into this account if the associated Dimension does not have a Value. There is more capability around this, but in terms of addressing your issue this is likely the setting used.

Hope this helps resolve your issues.