Which Ryzen 7? Which motherboard model?
You might need a BIOS update. Many MSI motherboards come with Flash BIOS feature that allows BIOS updates without a CPU, memory or graphics card installed.
Could you post all the system components.
You could try one RAM module on DIMM A2 (2nd from CPU) and see if it boots.
Unless stated on the box, a B450 needs a BIOS update to support a 3000 series CPU like the Ryzen 7 3700x.
The good thing is that the MSI B450 Tomahawk Max has a Flash BIOS Button that allows BIOS updates
without installing a CPU, memory or graphics card or even with incompatible CPU.
Just head to MSI website, download the BIOS for your motherboard, extract the file, and place the BIOS file on a USB flash drive (FAT32).
Power off the computer.
Plug the USB flash drive to the USB port next to the Flash BIOS Button (black USB port on the back)
Press the Flash Bios Button for 3 seconds and release it.
Within 5 seconds you will see a red blinking LED light, right next to the button. The PSU will power on and you might see some LED lights on the motherboard.
Wait for the red blinking LED light to go out then wait 5 minutes and power off the PSU.
Disconnect the USB flash drive.
Power on the PSU and turn on the PC.
Be patience, since it could take several minutes for the motherboard to configure hardware and it could power cycle several times.