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](https://github.com/systemd/systemd/issues/8307).

See also the (recently updated) [man page for systemctl](https://www.freedesktop.org/software/systemd/man/systemctl.html#enable%20UNIT%E2%80%A6) 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.