Withdraw of After tax contribution to traditional ira taxable?

In early 2021,I did a $6000 after tax contribution to traditional IRA, then withdraw it within 30 days before 4/15/2021.  When I received the 1099R form and import to turbotax, turbotax said $6000 distribution is taxable. 

 

I thought this is withdraw of after tax contribution should not be tax again. Anyone can help me, thank you.