The manual is saying it supports DDR5 4800/4000 MHz and the website says one of the supported is 6000(O.C.) . Using Partspicker, I bought G.Skill Trident Z5 RGB Series (Intel XMP) 64GB (2 x 32GB) 288-Pin SDRAM DDR5 6000 because the site said the board could use it.
Will this be an issue? Is it only 6000 if it's a 4800/4000 that is overclocked?
Just make sure you're on the latest BIOS version for your motherboard, then drop in the sticks of ram as listed on the motherboard manual, then enable X.M.P. If that doesn't work, manually input the timings, frequency, voltage and timings as stated on the sticker of the ram kit.
DDR5 base speed is 4800, same as DDR4 was 2133. That can change with the cpu memory controller speeds, same as 3rd Gen Intel was 1333/1600 for the cpu, even though DDR3 base was 1066. That means that Any speed higher than the cpu memory controller is (OC).
So in your case, the default speed will be 4800, but if you set XMP, it'll change that to 6000 (OC).
It's not an issue. That's exactly what XMP is, eXtreme Memory Profile. It's a factory overclock. But, it's not a guarantee. OC of any kind is a gamble, but while the ram is rated and tested and certified to attain that speed, there's no guarantee that your cpu/motherboard combo can support it or will actually work with it.