Copy qopensslbackend.dll in a tls folder next to your .exe, and DLL in your PATH :

./myexec.exe
./libcrypto-1_1-x64.dll
./libssl-1_1-x64.dll
./tls/qopensslbackend.dll

@JPolo said in Unable to compile when using QCustomPlot:

I use Qt Creator 4.15.0 based on Qt 5.15.2 MSVC2019

That is not necesserilly Qt version you're using (this is the Qt version used to build QtCreator).
What Qt version did you install?

What is the first error message?
Did you do what the tutorial suggests: "To use the shared library in your application, set the define QCUSTOMPLOT_USE_LIBRARY before including the QCustomPlot header."?

Fixed. This errors was because of Qt 6. I downloaded and linked previous version of QT - 5.15, and this errors disappeared

https://bugreports.qt.io/browse/QTBUG-87735
bug fixed)
Fix Version/s: 6.0.0 RC

@feistykittykat Don't know if I'm allowed to bump this bad boy up. Been struggling to use PVRVFrame or something similar to get openGL found on my machine, but I have no idea how to set it up for QT. Still not even sure if that's the source of my problem.