why does turbotax, tax my stimulus check when I enter it

No.  You do not pay tax on your stimulus payment.

If you did not receive your first and/or second stimulus payment or if the amount received is not correct, you can claim it on your 2020 tax return as the Recovery Rebate Credit.

In TurboTax Online, to claim the Recovery Rebate credit please do the following:

1. Sign into your account and continue from where you left off
2. Click on Federal in the left-hand column, then on Federal Review on the top of the screen
3. On the next page titled Let's make sure you got the right stimulus amount, click on Continue
4. Follow the interview
5. TurboTax will determine whether you are entitled to any additional stimulus
6. Any stimulus amount remaining due to you will show as a credit on line 30 of your form 1040.