Fixing Sliding Door Weather Stripping
Normally on the rear of the sliding portion of the door is a flap that engages at the last moment as the door closes.  That may cover your gap.  If you don't have one, you can add it (search for "sliding door draft stopper").

But looks like you could cut a block of wood to fit the gap (or, fancy, 3D print it).  The strip insulation is available at most hardware stores.

But really, this looks like a manufacturing defect, not even an install defect.