The error is coming from the slot assignment strategy. Try one of the other dropdown options to see if it will work for your purposes.

As far as getting the timing to be 30 seconds apart, you can set the source to have an inter-arrival time of 30 seconds, the processor to take 30 seconds, have the items go to a queue in between the processor and elevator and wait for 30 seconds, change the load / unload time of the elevator. You could also use process flow to delay the movement of the box until 30 seconds have passed. Would any of those work for what you're wanting?
To get the items taken out in order, click on Elevator2 to bring up its properties, look for the "Dispatcher" section, and change the "Queue Strategy" to "No Queue Strategy - Straight FIFO" (where FIFO means first in, first out).