if i checked free turbotax why do I have to pay

You can file with TurboTax free edition on any platform if you qualify.   You must have a Simple form 1040 return only (no schedules except for Earned Income Tax Credit, Child Tax Credit and student loan interest) in order to qualify. 

~37% of taxpayers qualify to file with TurboTax Free Edition.  To start for free, select Start for free on the Free Edition Learn More page on our website.   

You can try to downgrade also.  How to downgrade in TurboTax Online

The Free Version covers most people who have a simple return to fill out. Think 1040 Short. If it is telling you that you have to upgrade, then something in your tax information moves you into a different version of Turbo Tax. You have input some info that the Free Version does not handle. Sorry. It happens.