e.g. to C:\Qt\5.12.6\msvc2017_64 or C:\Qt\5.12.6\msvc2017 depending on which platform you have chosen
If you cant find this option, simply add a new entry of type "path" manually
+ - Disable the LYX_EXTERNAL_DTL option
+
- Enable the LYX_INSTALL option and click Configure.
- Click Generate.