Troubleshooting best way flow clear 800 gallon sand filter

I have a best way flow clear 800 gallon sand filter. I had it off and drained for several weeks during the winter. I turn it back on today to ensure it’s hooked up right with the pool filled and then realized I need to backflow first. It only ran for about four seconds and The handle will now not engage to allow a change in function.

How do I fix this? Troubleshooting in the manual does not cover this.