Can somebody describe why I am able to combine ga:yearWeek dimensions with ga:adCost, ga:impressions and ga:adClicks Metrics, but I am getting this error if I combine with ga:isoYearIsoWeek
(400) Selected dimensions and metrics cannot be queried together.
I know there is something called "Valid Combinations", but why this combinations is invalid ?
There are a number of dimensions and metrics that IMO Google doesn't have enough data for to allow you to analyze them together. In the case of the ad dimensions the data is coming from ad words and they may not deliver enough information to allow isoYearIsoweek to be analyzed with them. Remember this is just my opinion.
The only way to know what dimensions and metrics you can query together is to either make a request and wait for the error you see above. Or to check dimensions and metrics explorer by checking the dimensions and metrics you want to request data for the ones that cant be combined will be grayed out.
I have been requesting for years that Google give us a way of programmatically detecting invalid combinations but at this time google hasn't released anything.