For an individual tax return to be electronically fileable, every individual on the return must have a Social Security Number (SSN) or an Individual Taxpayer Identification Number (ITIN) issued by the IRS.
A tax return that includes an individual lacking either an SSN or an ITIN must include Form W-7, Application for IRS Individual Taxpayer Identification Number for that individual, and the return plus Form W-7 will be paper filed to the appropriate IRS Service Center.
Form W-7 can also be filed in person at any IRS Tax Assistance Center in the United States or through an authorized acceptance agent. The IRS advises allowing at least 7 weeks for processing and 9 to 11 weeks if filing during tax season.
Additional Information:
Can Form W-7 Be Electronically Filed?
Form W-7 - Application For An ITIN / Taxpayers Without a Social Security Number