Hi @HaithamTa ,
The Dataset ID referred in classifications API documentation isn't the ID of a AEP Dataset, it is the ID of a Classification Set in Adobe Analytics.
Classification Sets can be accessed in AA UI by navigating to Components -> Classification Sets,

You can get the ID of a Classification Set through any of these methods,
- In Classification Sets UI, open the set that you want ID for, then in the page URL you'll have the ID as shown below,
- Use Classification API to GET all classification datasets for a RSID and retrieve the Dataset ID for the variable you need from API response similar to below one,
Cheers!