canon pro-100 paper in tray error

if anyone can help me, i would sure appreciate it. i have a canon pro-100 that I have previously used on a pc but i just got an imac and when i try to print an image from lightroom, it tells me that the paper tray needs to be filled when i have several sheets inserted. The only issue i can think is that i was never prompted to select what type of paper i am using other than photo and plain. i am attempting to print an 13x19 on canon plus semi-gloss. I am very confused as to what is going on.

oh... and i have the rear tray selected

Could be wrong but I thought with my Pro9000 mkII that for 13 x 19 it had to do the feed through the rear so it doesn't bend the paper. Is it possible that is the case or that there is an option for that and its selected?

I assume by rear tray you mean the top tray.

Not sure why you want to print rear-tray with the Canon semi-gloss, it'll print fine from the normal top feed. I only use the rear feed with baryta paper, or something very thick. To use the rear feed, make sure the top feed is closed, then open the rear feed (do not collapse it all the way down, that is only to assist with jams) and put in one sheet--it'll feed in to the print head before it actually is ready to print.