I'm on Trixie (next stable in a couple of weeks), and only build QWT myself 
from source:

 # https://qwt.sourceforge.io/qwtinstall.html
 # download the tar.bz2 from 
https://sourceforge.net/projects/qwt/files/qwt/6.3.0/
 # unpack and go into ~/git/qwt-6.3.0 dir
 # error: qmake: could not find a Qt installation of ''  -> 
https://askubuntu.com/questions/1460242/ubuntu-22-04-with-qt6-qmake-could-not-find-a-qt-installation-of?
 qtchooser -install qt6 $(which qmake6)
 export QT_SELECT=qt6
 # create Makefile
 /usr/bin/qmake qwt.pro
 # compile
 make
 # install (in /usr/local/qwt-6.3.0 )
 sudo make install

Then, leaving out a lot of stuff...:

mkdir buildqt6;cd buildqt6;ccmake -GNinja -DBUILD_TESTING=FALSE 
-DENABLE_TESTS=OFF -DWITH_SERVER=FALSE -DWITH_3D=FALSE 
-DCMAKE_INSTALL_PREFIX=/home/richard/bin/qgis_/qt6/debug -DWITH_QWTPOLAR=OFF 
-DCMAKE_BUILD_TYPE=Debug -DWITH_GRASS=FALSE -DBUILD_WITH_QT6=TRUE 
-DWITH_OAUTH2_PLUGIN=FALSE -DWITH_WITH_SERVER_PLUGINS=OFF 
-DWITH_STAGED_PLUGINS=OFF -DWITH_PDAL=OFF -DWITH_QTWEBKIT=OFF 
-DQWT_INCLUDE_DIR=/usr/local/qwt-6.3.0/include  
-DQWT_LIBRARY=/usr/local/qwt-6.3.0/lib/libqwt.so  ..

in the same git repo as my qt5 build, QGIS works.

I'm on python 3.13.5 here

Not sure if this helps...

Richard




On 21-07-2025 17:52, Alessandro Pasotti via QGIS-Developer wrote:

Thanks, I'm using Python 3.13 (that comes with that Ubuntu version).

Now I'm trying with vcpkg ...


On Mon, Jul 21, 2025 at 5:35 PM pathmapper <[email protected] 
<mailto:[email protected]>> wrote:

    Hi,

    I don't know what's wrong, but when I had similar problems, switching to
    Python 3.12 solved them for me.

    Cheers,
    Björn

    On 21.07.2025 16:58, Alessandro Pasotti via QGIS-Developer wrote:
     > Hi,
     >
     > I am trying to build QGIS with QT6 on Ubuntu (tried 24.10 and 25.04),
     > it builds without errors but when I run it, the following error
     > appears:
     >
     > Couldn't load PyQGIS.
     > Python support will be disabled.
     >
     > Traceback (most recent call last):
     >   File "", line 1, in
     >   File
     > 
"/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python/qgis/gui/__init__.py",
     > line 25, in
     >     from qgis._gui import *
     > RuntimeError: qgis._gui cannot import type '���A� ' from
     > PyQt6.QtCore
     >
     > Python version:
     > 3.13.3 (main, Jun 16 2025, 18:15:32) [GCC 14.2.0]
     >
     > QGIS version:
     > 3.99.0-Master 'Master', e717e3cd2b9
     >
     > Python path:
     > ['/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python',
     > '/home/ale/.local/share/QGIS/QGIS3/profiles/default/python',
     > '/home/ale/.local/share/QGIS/QGIS3/profiles/default/python/plugins',
     > '/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python/plugins',
     > '/home/ale/dev/gdal/desktop-install/local/lib/python3.13/dist-packages',
     > '/home/ale/dev/gdal/autotest/pymod', '/usr/lib/python313.zip',
     > '/usr/lib/python3.13', '/usr/lib/python3.13/lib-dynload',
     > '/usr/local/lib/python3.13/dist-packages',
     > '/usr/lib/python3/dist-packages']
     >
     > I checked the QGIS/build/Desktop_QT6-Debug/output/python/qgis content
     > and it looks legit.
     >
     > PyQt6 installation also looks ok, importing core works ok, only gui
     > does not work.
     >
     > Any idea about what's going wrong?
     >
     > --
     >
     > Alessandro Pasotti
     > QCooperative: www.qcooperative.net <http://www.qcooperative.net> [1]
     > ItOpen: www.itopen.it <http://www.itopen.it> [2]
     >
     > Links:
     > ------
     > [1] https://www.qcooperative.net <https://www.qcooperative.net>
     > [2] http://www.itopen.it <http://www.itopen.it>
     >
     > _______________________________________________
     > QGIS-Developer mailing list
     > [email protected] <mailto:[email protected]>
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--
Alessandro Pasotti
QCooperative: www.qcooperative.net <https://www.qcooperative.net>
ItOpen: www.itopen.it <http://www.itopen.it>

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