Just update to the latest BIOS, it will enable it correctly for you by default. https://www.msi.com/Motherboard/B450-GAMING-PLUS/support
1) Get the latest BIOS. It's the topmost one on the MSI support page for your board.
2) Extract the file and you will get a text file and the BIOS file. Put the BIOS file into the root folder of a USB stick/drive.
3) Enter the BIOS by pressing DEL during boot, go to "M-FLASH" in the BIOS.
4) Once M-Flash (the updater) is loaded, it will show a list of your drives. Select the USB stick and select the previously extracted BIOS file on there.
5) It will ask for confirmation and then update the BIOS. It's fully automatic from there, takes about two minutes.