Did you upgrade the LR5 catalog during installation of LR6
If so you should have a LR6 catalog with -2 added to the name.
Thanks for your quick response. I did the usual LR install that I have done with every other new version and so I will check to see if the catalog updated automatically. Thanks for your suggestion
In the directory containing your catalog (*.lrcat), see if there is a file with the ".lock" extension. If yes, delete the "*.lock" file.
Update: I re-installed LR6 since I did not see it in the Adobe folder with all the previous LR versions and thought there may have been an install error. At program launch I receive the same catalog error and LR6 is still not in the Adobe folder with the LR versions on my local drive and I cannot find it. I looked to see contact info direct to Adobe and so far I've had no luck other than the forums.
Previous versions were listed with their dot release numbers e.g.
Adobe Photoshop Lightroom 4.4
Adobe Photoshop Lightroom 5.7.1
The new version is under Adobe Lightroom (no number)
Look inside the Adobe Lightroom folder and find the application: lightroom
Try double-clicking on lightroom to launch or on Windows you can right-click and pin to the task bar and launch from there with a single click.
Thank you for your suggestion.
Update: I updated the 5.7 catalog to v6 and it is working. Thank you both for your suggestions and helpful hints. I prefer not to be part of CC at this point, which is why I purchased this incremental update from 5.7 in the event Adobe ceases to release non-Cloud versions.