The use of QuarkXPress versions 2016 as well as the XV4 version of Q++Studio require the installation of the 64-bit version of the Microsoft Visual C++ 2015 Run-Time Library.
Solution: To address this issue, sitting at the workstation whence the message came, locate and run the program vc2015_redist.x64.exe, located in the folder Q:\Q++ServerXV4\Setup\.
Alternate solution: Since build 16645, this message gives you the option of having Q++Studio install the missing run-times for you.
IMPORTANT: After running this EXE, make sure to restart (reboot) your computer.
The Microsoft Visual C++ 2015 Run-Time Library should be pre-installed in Windows 10 and above. However, you might still get this message on Windows 10 and above computers, in which case you should follow the instructions above.