Hi Bohao,
SimBiology's units functionality does not support embedding numeric multipliers like 10^15. And even after you address that issue, you will probably see that SimBiology warns you because there is no unit called "cell".
My recommendation is to define your own custom units (and possibly a custom unit prefix) to support this set of units. When you do this, your custom definitions are stored in a unit library rather than in your model. This means that if you share your model with someone else, you will also need to share any custom units or unit prefixes that this model references. You can read more about the SimBiology user-defined library here. To make my answer more concrete, here's sample code that defines a unit for cell and another unit for 10^15:
cell = sbiounit('cell','dimensionless');
times1e15 = sbiounit('times1e15','dimensionless',1e15);
sbioaddtolibrary(times1e15);
m2 = sbiomodel('CART1201');
P_IL6 = addparameter(m2, 'P_IL6');
P_IL6.Units = 'picogram/(microliter*times1e15*cell*cell*day)';
P_IL6.ConstantValue = false;
P_IL6.Notes = 'CART induced IL6 secretion';
