I have solved !!!!
This is my report.
1. Ubuntu Mate in my PI4 (I had the same issue with Ubuntu O/S and Raspberry OS).
2. My mount neq6 is connected to PI4 by a Serial -> USB cable (Prolific)
3. Arduino for focuser
4. Devices are connected to an USB Hub

Once I plug the devices (neq6 and arduino), launch ekos, the message reports the device cannot connect because the port is already used.

Solution:
by linux terminal the comand lsusb returns:
Bus 001 Device 006: ID 2341:0043 Arduino SA Uno R3 (CDC ACM)
Bus 001 Device 005: ID 2109:2817 VIA Labs, Inc. USB2.0 Hub
Bus 001 Device 004: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port / Mobile Action MA-8910P

Solution:
After reading "indilib.org/support/tutorials/157-persis...al-port-mapping.html", I have created a new file:

sudo vi /lib/udev/rules.d/99-focuser.rules

with this text:
# MoonliteFocuser udev rule
SUBSYSTEMS=="usb", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0043", MODE="0666", SYMLINK+="focuser"


Now, I unplug/plug the Arduino cable or better, I restart the PI4.

For your info, the prolific device is in the file
/lib/udev/rules.d/99-indi_auxiliary.rules

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