As per a google search on the motherboard web site : https://www.asus.com/us/Motherboards/PRIME-B250M-PLUS/specifications/
4 x DIMM, Max. 64GB, DDR4 2400/2133 MHz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)
** Due to Intel® chipset limitation, DDR4 2133MHz and higher memory modules on 6th Generation Intel® processors will run at the maximum transfer rate of DDR4 2133MHz.
** Refer to www.asus.com for the Memory QVL (Qualified Vendors Lists).
* Due to Intel® chipset limitation, DDR4 2400MHz memory frequency is only supported by 7th Generation Intel® processors. Higher memory modules will run at the maximum transfer rate of DDR4 2400MHz.