I think I should be exempt from tax but GTP says I have to pay tax, why?

Many Nonresident Aliens incorrectly believe that they will not pay ANY tax in the U.S.; however, all individuals who receive income in the U.S., regardless of their U.S. Tax Status, are required to pay tax, unless you qualify for certain deductions, allowances, or exemption based on an income tax treaty. The amount of tax you will owe, if any, depends on your particular situation, the amount of income you receive, and whether or not you qualify for an exemption from tax based on an income tax treaty, or the limited deductions and allowances available to a Nonresident Alien.