The unit file (in this case strelaysrv.service needs to be in a partition that is already mounted when systemd starts, which is not the case in your example, since in the comments you mentioned /src is a symlink to under /home which is in its own partition.
This comes up fairly often in systemd bug reports, for a recent one see here.
See also the (recently updated) man page for systemctl which states:
The file system where the linked unit files are located must be accessible when systemd is started (e.g. anything underneath /home or /var is not allowed, unless those directories are located on the root file system).
The recommended solution is for you to create a copy of strelaysrv.service under /etc/systemd/system rather than a symlink. That should fix the issue.