It seems like your project type setup for the Asset Allocation method has been defined as - Standard Unit Cost.
However, you may missed to define the unit standard cost for the asset book and category in the Project Assets Standard Unit Cost window.
Look at the following explanation in Projects Implementation Guide:
You can set up a capital project to automatically allocate unassigned and common costs to multiple assets by selecting an asset cost allocation method for the project. To allocate
costs using the Standard Unit Cost method, you must define a standard unit cost for each asset book and asset category combination for which you want to allocate costs. When
you choose this method of cost allocation, Oracle Projects multiplies the standard unit cost times the units installed for each asset to determine the proration basis for