Installation not allowed by ".." slot rule of interface "serial-port"

Retryed now like this:

$ sudo snap set system experimental.hotplug=true
$ sudo systemctl restart snapd
$ snap interface serial-port
name:    serial-port
summary: allows accessing a specific serial port
plugs:
  - test
slots:
  - pi:bt-serial
  - snapd:pl2303serialport (allows accessing a specific serial port) <- it appears

I also have a udev rule:

$ cat /etc/udev/rules.d/icse012a.rules 
SUBSYSTEM=="tty", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", ACTION=="add", RUN+="/home/user/icse012a-init"

The devices appeared as snapd:pl2303serialport and it responded ( i heard the relay work ).

Created snap for armfh on lxc: test.snap on RPi3 B+ as explained here: https://ograblog.wordpress.com/2020/07/10/building-snap-packages-on-ubuntu-core/

Then:

snap install --dangerous ./test.snap 

Now the eletron app in the snap test is displayed with the browser console enabled, and with the error:

Error: Command failed: echo '50' | xxd -r p >>/dev/ttyUSB0
/bin/sh: 1: cannot create /dev/ttyUSB0: permission denied.

Then:

$ snap connections test
Interface         Plug                   Slot               Notes
browser-support   test:browser-support   :browser-support   -
hardware-observe  test:hardware-observe  -                  -
home              test:home              -                  -
network           test:network           :network           -
network-bind      test:network-bind      :network-bind      -
opengl            test:opengl            :opengl            -
pulseaudio        test:pulseaudio        -                  -
raw-usb           test:raw-usb           -                  -
serial-port       test:serial-port       -                  -
wayland           test:wayland           mir-kiosk:wayland  -

$ snap connect test:serial-port snapd:pl2303serialport

$ snap connections test
Interface         Plug                   Slot               Notes
browser-support   test:browser-support   :browser-support   -
hardware-observe  test:hardware-observe  -                  -
home              test:home              -                  -
network           test:network           :network           -
network-bind      test:network-bind      :network-bind      -
opengl            test:opengl            :opengl            -
pulseaudio        test:pulseaudio        -                  -
raw-usb           test:raw-usb           -                  -
serial-port       test:serial-port       :pl2303serialport  manual
wayland           test:wayland           mir-kiosk:wayland  -

$ snap restart test
Restarted.

The electron app opens with the same error as before: permission denied.

Any thoughts?

EDIT:

In this version i removed the slot for serial-port interface from snapcraft.yaml !!!