My son lived and worked in Singapore in 2018 with foreign income and also in California. Turotax won't let us file a state return?

My some worked in CA from January through August 2018 and got a W2 and paid state taxes.  Then he moved to Singapore from Sept to December with foreign income.  In trying to do his taxes, Turbotax says he does not need to file a CA state return and it won't even allow us to.  I am sure he has to file a state return.  How can  we do it?