If you encounter the error as below:
INFO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/upf_init.c:164 UpfInit() Config: /home/free5gc/go/src/free5gc/src/upf/build/config/upfcfg.test.yaml UPF=Util
INFO[2020-06-06T13:45:11Z] Set log level: info UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/lib/gtpv1/src/gtp_link.c:27 GtpLinkCreate() GTP device named upfgtp0 Create fail UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/lib/gtpv1/src/gtp_path.c:49 GtpTunCreate() GtpLinkCreate fail UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/lib/gtpv1/src/gtp_path.c:144 GtpDevListCreate() GTPv1 tunnel create fail : IP[10.200.200.102], ifname[upfgtp0] UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/up/up_path.c:74 GTPv1ServerInit() GtpLinkListCreate Fail UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/upf_init.c:170 UpfInit() UPF - GTP-U Server error when UPF initializes UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/upf.c:28 main() UPF failed to initialize UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/up/up_path.c:233 BufferServerTerminate() epoll deregister error UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/upf_init.c:183 UpfTerm() UPF - Buffer Server error when UPF terminates UPF=Util
ERRO[2020-06-06T13:45:11Z] /home/free5gc/go/src/free5gc/src/upf/src/upf.c:36 main() UPF terminate error UPF=Util
Cannot find device "upfgtp0"
This is most of the time caused by gtp5g module not being inserted.
Please notice that this kernel module will not be automatically loaded when booting. You will need to install it manually every time when you reboot the vm/system.