Hey, I accidentally marked that I can be claimed as a dependent on my return. I learned after filing that my parents won't claim me. Will this affect my stimulus check?

Yes, it will affect your stimulus check.  You would not be eligible for a stimulus payment.

According to the information on the IRS site, if you were claimed as a dependent or can be claimed as a dependent, then you will not be eligible for a stimulus payment.  Having checked the box on your return that you could be claimed as a dependent will disqualify you from receiving a stimulus payment.  

IRS - Economic Impact Payment

@bloomj

I accidentally marked that I can be claimed. If I file an amended tax return, will I then qualify for the stimulus payment?

You should qualify for a stimulus payment once this is corrected.