Do I need to file a Form 8275/Schedule 1 to explain why all of 1099-K income was a personal transaction?

Form 8275 is not required.  When reporting the IRS form 1099-K income, report the incorrect amount in the box that states:

"This amount in box 1a is too high or includes some personal transactions."

The amount in error will be reported at the top of Schedule 1 Form 1040.  Note the amount reported in error or for personal items sold at a loss.

Not sure what kind of income you are reporting for the IRS form 1099-K.  To access the form in TurboTax Online, follow these steps:

• Down the left side of the screen, click on Tax Home.
• Click on the down arrow to the right of Review.
• Click on the blue band labeled Review.
• Down the left side of the screen, click Federal.
• Down the left side of the screen, click Wages & Income.
• Scroll down to Other Common Income click on the dropdown menu.
• Under Your income and expenses, click the Start/Revisit button to the right of Form 1099-K.
• At the screen Did you get a 1099-K? click Yes.
• At the screen How would you like to enter your 1099-K? click Type it in myself.  Click Continue.
• At the screen Choose which type of income your 1099-K is for, select the appropriate button.  Click Continue.
• At the screen Let’s get the info from your 1099-K, enter the information. 
• If necessary, note the box "This amount in box 1a is too high or includes some personal transactions."
• Click Continue.

Thank you. It was a personal transaction through Venmo accidentally marked as a business transaction, so it's really not reportable income at all. Venmo said they would not issue a corrected 1099-K. 

To avoid IRS issues, go ahead and enter Form 1099-K with the adjustment as JamesG1 posted earlier. No further explanation is needed.