If you are a US citizen or US resident alien and your gross income is at least the minimum for your filing status then you are required to file a tax return.  See IRS Publication 501 page 2 - https://www.irs.gov/pub/irs-pdf/p501.pdf

If you are legally married and living with your spouse at the end of the tax year then you can only file as either Married Filing Jointly or Married Filing Separately.   If your spouse doesn't have and isn't required to have
an SSN or ITIN, enter “NRA” in the space for your spouse's SSN.  You will have to file as Married Filing Separately. 

Where asked to enter the spouse's Social Security number enter 999-88-9999.  You can only print and mail your tax return, it cannot be e-filed.  When you print the tax return erase the Social Security number for your spouse and manually enter NRA.

See this TurboTax support FAQ for the procedure to print and mail a tax return using the online editions - https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online

We are not living together at the moment. I do not know if she is required to have an SSN or an ITIN. 

Do you have a qualified dependent living with you in 2018?  Did you live with your spouse in the same household at any time during the last six months of 2018?

Questions asked to determine if you could file as Head of Household where the spouse is not reported on your tax return.

If you are legally married and do not have a qualified person as a dependent then you can only file as Married Filing Separately if your spouse does not have a SSN or an ITIN and you are not applying for an ITIN with your tax return,