I am an F-1 student in my 7th year, but I spent all 2018 outside the US (in my home country). Am I considered resident or nonresident alien for tax purposes for 2018?

It seems like you did not meet the substantial presence test and therefore you are considered a non-resident. 

You will be considered a United States resident for tax purposes if you meet the substantial presence test for the calendar year. To meet this test, you must be physically present in the United States (U.S.) on at least:

1. 31 days during the current year, and
2. 183 days during the 3-year period that includes the current year and the 2 years immediately before that, counting:

• All the days you were present in the current year, and
• 1/3 of the days you were present in the first year before the current year, and
• 1/6 of the days you were present in the second year before the current year.

You can only count the days after the 5th year because you were exempt for 5 years on an F1 visa.

Thank for the answer. At this point though I don't understand why the TurboTax software, having this piece of information allowed me to move on. I have already paid but I now find myself with the wrong form. I have a 1040 instead of a 1040NR. What am I supposed to do? I don't think paying twice is fair, which is what I have to do if I now try to file with Sprintax.

Please try calling TurboTax support at (800) 446-8848.