How do I create expense asset for accrual at receipt?

Summary:

I would like to know how to create expense asset from PO/AP using Accrual at Receipt method.

I have created PO to matching with AP invoice. But in the AP Invoice Distribution, I was not able to check 'Track as Asset' box for Accrual at Receipt. The box is gray out and can not update anymore. 

However, if the Accrual method was set as Accrual at Period End, I was able to check 'Track as Asset' box.

Any steps that I am missing when create PO using Acrcual at Receipt method? Please advise.

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