Try entering '000 000 000' for your spouse's SIN, this should let you continue. Or, enter your marital status as 'single' for now, then continue your preparation, but do not file. When you can access your spouse's SIN, go back and enter that in.
It is best not to file your return as single when you have a spouse as there are certain credits such as pooling medical expenses or charitable donations that can result in a larger refund or lower tax owing by filing together.
Best,
Julie