My new LR6 opens LR4 catalog and also, does not recognize Canon SL-1.  How do I fix this?

LR4 and LR6 are a side-by-side install. Make sure you are starting the correct version. Often times your desktop icons do not get changed from LR4 to LR6.

LR6 cannot directly use your LR4 catalog. It must migrate the data to a new LR6 catalog. It will append a "-2" to the end of the file name on the new catalog.