Figured I would just share this for future users who might encounter the same issue. I tried to install Tailscale on my RPi 3 (Raspbian Buster) and go the following error:
Jun 15 15:18:55 raspberrypi tailscaled[19453]: is CONFIG_TUN enabled in your kernel? `modprobe tun` failed with: modprobe: ERROR: ../libkmod/libkmod.c:586 kmod_sear
Jun 15 15:18:55 raspberrypi tailscaled[19453]: modprobe: FATAL: Module tun not found in directory /lib/modules/5.10.11-v7+
It transpires that an existing Wireguard install (using PiVPN) was using TUN. Uninstalling it allowed Tailscale to install with no issues.