Avnet has a set of build scripts you can use to build a known-good hardware platform and PetaLinux BSP with the 2018.3 Vivado and PetaLinux tools. You will need to clone Avnet hdl, bdf, and petalinux git repositories and checkout the named branches:
https://github.com/Avnet/petalinux (2018.3 branch)
https://github.com/Avnet/hdl (2018.3 branch)
https://github.com/Avnet/bdf (master branch)
You will need to download/clone them into the same parent folder, as the build scripts rely on relative paths. To build the 2018.3 BSP for the Ultra96-V2 board you can run the petalinux/scripts/make_ultra96v2_oob_bsp.sh script. The build script takes care of fetching the WiFi drivers, etc. This will also build the Vivado project for the programmable logic and processing system configuration.
All of this of course assumes that your Vivado and PetaLinux tools are installed and licensed, etc. Good luck!