I’d check the Object Series associated with the fixed asset group for which the PO is made. Then Go to “Object Series” window, RMB to Object Number Series and check that there are valid asset series for the current year. 

there is no series

You need to add the numbering series there.

When the FA is set up to automatically create the asset from the PO receipt, the series has to be there so that IFS has something to put in the ID. The asset ID is a concatenation of the series ID and the number defined. (In your example it will be CASxxxxx).