because I get a serial number of lightroom 6. and I have found if I want to only use lightroom 6 must to cancle the crative cloud membership. so what can I do
You can use Lightroom 6 standalone (without Creative Cloud) if you purchased it.
If you used a trial of Lightroom 6 and then purchased it to use as a standalone product, please see this link to serialize it.:
Is this what you were trying to do?