Hello taiquyet2024,   The R640 can support U.2 drive in certain configuration. I will note that field changing of backplane configuration is not supported. There are not kits available for conversion.   Check if you have x10 backplane. Only X10 backplane support NVMe drives. Backplane details [URL]   Check your cabling: Cable routing [URL]   Controller S140 (software controller) NVMe PCIe SSD support [URL]  
I can mount a U2 SSD hard drive for the server R640 ?  
Dell EMC PowerEdge R640