I am self-employed. Where on Schedule C should I enter the standard deduction for Meals

Please see this answer from @kll.

Per Diem goes in the Meals and Entertainment area.  While inside the software and working on your return, type Schedule C in the Search at the top of the screen (you may see a magnifying glass there).  There will be a popup that says Jump to Schedule C.  Select that to get to the general area. 

• then, Business Income and Expenses
• select Edit next to your business (if it is there)
• scroll to Business Expenses
• then, Other Common Business Expenses
• select that, then on the next screen you will see Meals and Entertainment

You can find the current Per Diem rates for the location to which you traveled at this link Per Diem.  Remember to select the correct tax year (2021) from the drop-down menu.