Why Dell keep placing a service hold on a system?

The most common cause is a system not registered to the person making the warranty claim. 

Are you registered as the owner with Dell?  If the system was purchased through a third party (reseller, used, Amazon storefront, etc.) you will need the assistance of the seller to transfer ownership.

But I already transfered the warranty before they placed the hold on the tag. 

You'll need to ask Dell about your case, but note that transferring ownership and transferring a warranty are distinct.  They are not the same thing.

A system purchased in one country can have ownership transferred, but a warranty transfer will be denied if it's a basic 12-month warranty (not applicable out of the country of purchase).  Another recurring one is if a system was initially sold in a country where Dell is now prohibited from doing business (there are two in Europe) -- those systems are orphans now, having no Dell support.