“Phantom inventory” refers to advertising inventory that appears to be available for purchase but is actually non-existent or unavailable for various reasons.
Do you have enough web inventory and your campaigns do not deliver?
These differences between the actual inventory of your website and the advertising inventory can have different causes:

Implementation errors in the adserver.

Incorrectly declared advertising spaces.

Positions that are not “painted”.

Wrong nomenclature in the positions.

How to detect it?
The most convenient is to carry out a complete audit of the advertising inventory and its implementation. The main solutions are:

Define the advertising inventory.
Parameterize the adserver according to the advertising strategy.
Review all positions on different devices.
Assign house campaigns to all positions.
Use intelligible and standardized nomenclature.
Check that all campaigns are trafficked on the audited positions.