Preparation:

installed VisualStudio 2019

installed prebuild binary Qt 5.12.7 VS2017 x64

(as a second test scenario: source build QT 5.12.8 with visual studio 2019 command line x64)

Test:

started Developer Console

changed to folder with QtCreator default created Qt Widget App (which compiles with QtCreator with VS2019 btw.).

Extended PATH to have qmake:

C:\Users\IGI\Documents\QtProjects\Test_5_12_8__N1>qmake -v
QMake version 3.1
Using Qt version 5.12.7 in C:/Qt/Qt5.12.7/5.12.7/msvc2017/lib

now calling qmake with parameters to have a sln file for VS2019 

C:\Users\IGI\Documents\QtProjects\Test_5_12_8__N1>qmake -tp VC Test_5_12_8__N1.pro -o Test.sln
WARNING: Unable to generate output for: C:/Users/IGI/Documents/QtProjects/Test_5_12_8__N1/Test.sln.Release [TEMPLATE VCapp]
QIODevice::write: device not ope
WARNING: Unable to generate output for: C:/Users/IGI/Documents/QtProjects/Test_5_12_8__N1/Test.sln.Debug [TEMPLATE VCapp]
QIODevice::write: device not open
 

An sln file is created, but opening this with VS2019 results in an error message:

"The selected file is not a solution file."

and the file can not be used.

Where is my mistake?