I see that development is appearing to resume on this repo. As a quick fix for some newer Qt5 versions,
there are issues with it finding QOpenGLPaintDevice on newer Qt5 versions. This is because of a missing include in the < Qt6 block. I'm not sure which version of Qt5 caused this change, but there's likely a shift in these include requirements in some version of Qt5 that is not accounted for.

#ifdef QCP_OPENGL_FBO
#  include <QtGui/QOpenGLContext>
#  if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
#    include <QtGui/QOpenGLFramebufferObject>
#  else
#    include <QOpenGLFramebufferObject>
#    include <QOpenGLPaintDevice>
#  endif
#  ifdef QCP_OPENGL_OFFSCREENSURFACE
#    include <QtGui/QOffscreenSurface>
#  else
#    include <QtGui/QWindow>
#  endif
#endif

should be

#ifdef QCP_OPENGL_FBO
#  include <QtGui/QOpenGLContext>
#  if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
#    include <QtGui/QOpenGLFramebufferObject>
#    include <QtGui/QOpenGLPaintDevice>
#  else
#    include <QOpenGLFramebufferObject>
#    include <QOpenGLPaintDevice>
#  endif
#  ifdef QCP_OPENGL_OFFSCREENSURFACE
#    include <QtGui/QOffscreenSurface>
#  else
#    include <QtGui/QWindow>
#  endif
#endif