Issue 'File not found' but file exists ("Undefined reference")

[Dane 0]

Hi! I am new to Qt and trying to get my first app going on an embedded board (i.MX6ULL ARM).
I have a basic Qt Quick app going on the host Linux machine (ok WSL...), but am struggling to get it cross-compiled for the embedded board.
I am using Buildroot to build the system image.
I have added the Buildroot cross-compilers from buildroot/output/host/usr/bin/ to Qt under Kits -> Compilers and also added the version (qmake) from the same directory.
I have added a Kit with "Remote Linux Device" device type.
When I try to build for the embedded device then I get 102 x "undefined reference to" issues mostly relating to libQt5Core.so.5.15.8 and libQt5Gui.so.5.15.8 (but also 2 issues from libQt5Network.so.5.15.8).
The issues relating to libQt5Core.so.5.15.8 state "File not found: /home/dane/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/usr/lib/libQt5Core.so.5.15.8" however I have checked and verified that the file is there. The other issues (Gui and Network) do not complain that the file is not found, but still complain regarding "undefined reference".
How do I resolve this?

[mvuori]

I don't know, but perhaps the files lack read permissions for the build tools.