***Follow this advice at your own risk, I am just a novice overclocker/tweaker sharing my experience. There are many other guides online on how to do this ***
Yes, it is possible to install Windows 7 on the x370-a with Bristol ridge APU but I would not advise it due to lack of support, i.e. updates. I own the x370-A and R5 1600 and a A6-9600 and have installed and used windows 7 on both for benchmark purposes.The experience was the same for both. I have installed Win 7 using 2 methods multiple times. It was moderately difficult to get it to install and work. Before you start make sure to go into BIOS/UEFI, Advanced view, Boot options, Secure boot and change to "Other OS" save and exit.
(1)On the product page of the x370-A choose specifications. Then the support tab, next choose the drivers and tools tab. It will ask which OS you want for BIOS/firmware and drivers. Choose Windows 7 32/64(whichever applies), under the utilities option there is a tool labeled "ASUS EZ Installer Use this tool V1.03.16 to create a Windows 7 installation file with USB 3.0 drivers preloaded". Once you created a new USB ISO with this utility it will install "normally" (a lot of different variables can cause issue)
**look at motherboard manual to find which USB ports are USB 2.0 this is the port needed for install of Win 7 OS
(2) ***You will need a PS/2 port keyboard and a another PC that can run Win 7 natively
1.Install Windows 7 on the system without USB/chipset support (x370-A and APU).
2.Once install is complete you will be unable to use mouse or keyboard, power down and move this hdd/ssd to another windows 7 compatible system.
3.Once you are in the compatible Win 7 system you can use mouse and keyboard, download the USB 3.0 drivers and chipset and put these on your desktop.
4. Move hdd/ssd back to the x370-a and use the PS/2 keyboard and the TAB button to navigate on the desktop.
5. Run/install drivers and chipset and restart. After restart the machine should be running Windows 7 normally. (after updating you will get constant popups about Win 7 incompatibility with your system)
As to the "this bios is not fully ACPI compliant" error message. I have not seen this error when I installed Win 7. I have had various other issues during Win 7 installs on Ryzen/Bristol Ridge but none that I was not able to worked around. I did this on BIOS/UEFI 3401 and 3803 not sure about newer BIOS as I am no longer using this PC configuration.