Help to setup QT5CT?

[SignorCastello]

the error is this. so how can I solve that??

[zabadabadoo]

Changed title, as "How to" is generally used to describe tutorials.

[SignorCastello]

ok sorry

[zabadabadoo]

No apology needed. I was just stating reason for the edit.

I am not familiar with QT5CT, but did a websearch and found this:

GTK theme does not work properly on Qt Startup Applications

Don't know if it is any help to you.

[Aravisian]

Yes, I have gotten this warning too. I know the feeling.
Elevate to Root with your file manager:

sudo -i

thunar

Hit the up arrow on the toolbar three times to get to the top level of the tree.
Now, navgate to /etc/profile.d/
You will see Two Files in here.
qt-qpa-platformtheme.sh
and
qt-style-override.sh

I blanked out qt-style-override.sh and saved it as an Empty File.
In qt-qpa-platformtheme.sh, ensure the contents say only

export QT_QPA_PLATFORMTHEME=qt5ct

and ensure the file is saved.

Now, navigate to /etc/X11/Xsession.d and open 99qt5ct with text editor.
The contents should be:

# Enable qt5ct by default unless another PLATFORMTHEME option is set
# or KDE is running.
if [ -z "$QT_QPA_PLATFORMTHEME" ] && [ "$XDG_CURRENT_DESKTOP" != "KDE" ]
then
        export QT_QPA_PLATFORMTHEME=qt5ct
fi

ensure that it is exact, please.

Next
Navigate to /etc/X11/Xsession.d and open 56zorin-os-lite-session

The contents should

# Set default environment vars in Zorin OS Lite
if [ "$DESKTOP_SESSION" = zorin-os-lite ]; then
    # QT5 apps to use GTK style
    export QT_QPA_PLATFORMTHEME=qt5ct
    # Workaround clutter issue (LP: #1462445)
    export CLUTTER_BACKEND=x11
fi

Ensure it is exact and file saved.

That should be everything.
Do a hard reboot and test.