About findfs: unable to resolve 'LABEL=writable'

hi,ogra,thank you for the reply… …
I have some new questions about system user generate…
i put auto-import.assertion/auto-import.assert in /writable and snap known system-user show it have made a new system user, but when i reboot ,it still show

Ubuntu Core 16 on <no ip address> (ttymxc0)                                     
                                                                            
You cannot log in until the system has an IP address.                           
(Is there supposed to be a DHCP server running on your network?) 

I use make-system-user.run to make the auto-import.assert, Do i miss some information in document?

and there is another error about Initialize device

21   Error   today at 11:13 UTC  today at 11:13 UTC  Initialize device
22   Error   today at 11:15 UTC  today at 11:15 UTC  Initialize device
root@localhost:~# snap change 22
Status  Spawn               Ready               Summary
Done    today at 11:15 UTC  today at 11:15 UTC  Generate device key
Error   today at 11:15 UTC  today at 11:15 UTC  Request device serial

......................................................................
Request device serial

2018-10-29T11:15:37Z ERROR cannot deliver device serial request: Cannot process serial request for device with brand "my brand id" and model "tsimx6"

refer to this,i have add slot interface in gadget. should i connect kernel.snap interface to gadget snap?

and in this post about interface,how do those interface work without defined in snap?and how can i make these interface work?