Did you prepare the non-resident state return? You should prepare that return first and then the information will transfer to the "other state tax credit" topic on your resident state return.
You need to enter the amount of tax paid to the nonresident state and the only way to know that number is to prepare the non-resident state return first and then complete your resident state return.
If you lived in Virginia, you claim the other state tax credit on the VA return. You will need to enter the amount of income earned in CA (reported on the W2) and the total tax for CA (the value from the CA tax summary). The credit is calculated using those values -- your VA tax will be reduced by the amount of tax you paid to CA on the same income.
So go to your CA return and enter the amount of total income for VA (from the VA tax summary) and the total tax for VA.