I guess by canal you mean the Aire, which is navigable. The Selby canal joins the Aire.
It's possible to cycle a short leg of the Selby canal from West Haddlesey but before long there's a "no cycling" sign. In any case you can't reach the Selby canal along the Aire towpath, as far as I know. It's incomplete or it's a footpath only.
There is a way, but not entirely along the navigation: Take Route 67 from Leeds city centre as far as Mickletown, but make sure you cross to the north side of the Aire, then you would take yellow roads through Allerton Bywater, Fairburn, Burton Salmon, Gateforth, Burn and join the TPT to get to Selby. I have cycled that route so I know it's do-able.