Installation of "Oracle Developer Tools for Visual Studio 2017" version 12.2.0.11 with MS Visual Studio 15.6.7 fails with
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Pre-check verification: Your machine is busy with another installation.
...
in Oracle.VsDevTools logfile.
It seems that executing the VSIX installer manually (command line parameters from Oracle.VsDevTools log) is working.
There must be some changes in the MSI/VSIX execution process in VS 15.6. (MSI package calls VSIX installer which calls another MSI package or something like that.)
HTH
Andreas