Are there any other usb sticks attached to the machine? This poster had a really hard time getting their zstick to work but they also had a zigbee stick. Ultimately they needed to a UDEV rule and move the zstick up to a higher ttyacm#
See this thread as well
Synology has a locked down OS you can’t make many changes to and are trying to workaround a lot. As you can see by the headaches in those attached posts, along with many other people having problems with synology and zwave and zigbee sticks, I personally would avoid running Home Assistant on synology if you want to also use zwave and/or zigbee sticks.
If you want to keep trying though, per this post, try and install the container through the command line instead of the synology UI: