You can only archive purchase orders that are completed.
If the received quantity is less than the ordered quantity, then SAP cannot know that this order is completed, hence it needs an indicator set by a human that explains that no further receipt is expected.
So you are either archiving too early and you have to be glad that SAP does these checks or someone has to set the delivery complete indicator, usually through the final entry sheet indicator.
for detailed understanding see OSS note 735404 - Delivery completed indicator, final entry sheet indicator
The second seems to be obvious, the values are not in balance and you had not yet done MR11.
The last point is usually from contracts and schedule agreements where you defined yourself additional checks in customizing, go SARA, enter MM_EKKO, click Customizing button and execute the last 2 tasks in section application specific customizing and check the definition.