I went to college last year and they didn't issue a 1098-T because I received grants. How can I claim the American Opportunity Credit? I used all of the grant on school.

You can still claim the AOTC if you haven’t received a Form 1098-T because your school isn’t required to provide you one, if:

• You meet all other eligibility requirements for the credit, 

• You can show you were enrolled at an eligible educational institution, and 

• You can substantiate the payment of qualified tuition and related expenses.

See Education Credits: Questions and Answers for details.

To enter your education information in TurboTax Online, 

1. Click Pick up where you left off

2. Click Federal in left pane

3. Click Deductions & Credits tab at top

4. Scroll down to Education and click the Show more dropdown

5. Answer No on the Did you get a 1098-T for the year 2021? Page

6. Check the I qualify for an exception box on the OK, let's see if you qualify for an exception page

7. From there, you can enter your relevant information.

It doesn't matter if your parents are actually going to claim you or not.  If you qualify as their dependent, you are not allowed to claim a tuition credit*. I does appear that you qualify as their dependent.

There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A full time unmarried student, under age 24, even if you don't qualify as a dependent, is only eligible for the refundable portion of the American Opportunity Credit  (AOTC) if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. You usually must have actually paid tuition, not had it paid by scholarships & grants.  It is usually best if the parent claims that credit. 

You cannot claim a credit if you are, or can be, claimed as a dependent by someone else.

Reference: Line 7 instructions for form 8863. https://www.irs.gov/instructions/i8863

*If the student actually has a tax liability, there is a provision to allow him to claim a non-refundable tuition credit. But then the parent must forgo claiming the student as a dependent, and the $500 other dependent credit.  The student must still indicate that he can be claimed as a dependent, on his return. This is worth up to $2500 (AOTC shifts to all non refundable)