Hi Callum,
Yes that path is hardcoded in the code, and you need to have that path manually created and properly chown'ed before using it with osmo-gsm-tester. See https://gitea.osmocom.org/osmocom/docker-playground/src/branch/master/osmo-g... on the steps required to set it up. You can also have a look a osmo-gsm-tester User Manual for info in general too: https://ftp.osmocom.org/docs/latest/osmo-gsm-tester-manual.pdf
Regards, Pau