Supply voltage drops AFTER connecting to device
What is the cause? and where is the cause.


Impossible to know based on the information given. It could be any number of reasons. Most likely, you're triggering a resettable fuse in the power adapter due to an over-current situation when you plug it into your laptop. Possible causes could be:


The power adapter's plug has reverse polarity relative to the laptop.
The power adapter is faulty and is shorting internally.
The laptop's power receptacle is faulty (maybe a piece of metal is jammed in there or a pin is bent).


Also, your power numbers are a little funny. A power supply rated at 19V and 2.1A has a maximum capacity of 39.9W, not 45W.