Wireguard - Kernel headers don't seem to be available, can't compile the module. Sleeping now

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Raspian version 10 (buster)
uname -a gives:
Linux raspberrypi4 5.4.49-v7l+ #1323 SMP Fri Jun 26 14:40:18 BST 2020 armv7l GNU/Linux
mark@raspberrypi4