Storage units upper and lower bound definitions
In storage units line 67, the upper and lower bounds of the SOC are respectively defined as soc_mincapacity and soc_maxcapacity. Generally speaking the user will define the capacity, but when the objective is to minimize the capacity (such as in the storage_design example), the capacity remains None, and so these upper and lower bounds of the SOC cannot be defined.
An alternative upper and lower bound version should be coded for the SOC.