Of the total amount of grids, one grid is called a Partial grid. That is because it's size is not as wide as the others. And if you inspect the arrangement or orientation of the grids is they go around in a circle on the grid manifold, you will see that the hole that the standup pipe goes to, will be right in the way, if you place a full-size grid there. So, as you begin to place the grids into the manifold, start with the partial grid right next to that hole. Continue going all the way around, and then place the grid retainer on top, and then the long threaded rods to tighten them together. Some patience, and careful rearranging, and manipulation, will get the retainer on top of the grids just right.
If you look at the picture in the link that I posted below, you will see that protruding section of the manifold, (right in front), where the hole is for the standup pipe, that is where the partial grid will go/start.
Pentair 59023400 Complete Element Grid Assembly Replacement 48 Square Feet...

Start with the short one by where cut out is for pipe arrange then put back together see manual if you dont have manual check pentair site or send me more info ill try to help