If I am still legally married but have not been in contact with my spouse for over 15 years how do file cause I do not know his SSN or current address?

[mianari]

I am trying to file married but filing separately but it keeps asking me for his address, SSN, and if w-2

[Bsch4477]

If you have a qualifying child you can file as Head of household. If you have to file separately you will have to mail your return. 

[Mike9241]

 if you paid more than 1/2 the cost of keeping up a home for yourself and your child who lived with you for more than 6 months of the year you can file as head of household.  your spouse's info is not needed.

"your child" means your child, stepchild, or foster child who you can claim as a dependent.

so the child if not a full-time student must be under 19 at the end of the tax year or if a full-time student must be under 24 at the end of the tax year. The child hasn't provided over ½ their own support. The child didn't file a joint return unless there was no tax liability but merely filing jointly to facilitate the refund of taxes withheld or estimates paid

if you qualify for HOH then you can e-file without your spouse's info.

if you don't qualify as HOH, then without thee spouse's info you can not e-file. you'll have to file by mail and include the explanation you gave.

[Opus 17]

As stated, if you provided care in your home for a  qualifying person, usually your child dependent, then you should qualify for head of household status and don’t need your spouses Social Security number.  (If you have not been troubled by lack of Social Security number before, I wonder if you might have had a dependent in the past who is not a dependent this year.)

if you don’t qualify for head of household, you must file as married filing separately. You can’t e-file if you don’t have the Social Security number. You have to print your return and mail it in, and attach a written statement giving your spouse‘s name, last known address, identifying information that you do know such as a birthdate, and explain why you are unable to provide the Social Security number.

you might have to provide the program with a fake Social Security number before you can print your tax return. You can use 999-88-9999, but you must remember to white this out on the printed copy before you mail it.