I have mega backdoor roth for 2023 (already filed) and 2024 and backdoor roth for 2024. The 1099 R generated includes all, causing an excess traditional IRA contribution.

You did something wrong, not sure what.  However, a rollover (which is what a backdoor Roth is) is NOT a contribution.  Don't enter any IRA contributions unless you actually made contributions separate from the conversion.

For the conversion, only enter the 1099-R from the originating bank or broker.  When Turbotax asks what you did with the money, one of the choices should be a rollover or Roth conversion.  Don't enter the amount again as a contribution. 

I checked the option - I converted some or all of it to a Roth IRA when asked with the question - 
"Tell us if xxx moved the money through a rollover or a conversion"

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@saket1906 wrote:

I checked the option - I converted some or all of it to a Roth IRA when asked with the question - 
"Tell us if xxx moved the money through a rollover or a conversion"

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That's fine.  But you will never get a message about "excess contribution" from a rollover or conversion, no matter how big.  You must also have entered a contribution in the IRA deduction section. 

You're correct. In the Deductions and Credits section,

- Under 'Tell Us How Much You Contributed' - I mentioned the entire value of 2023 megabackdoor roth, 2024 mega backdoor roth and 2024 backdoor roth. 

- Then in 'Any Nondeductible Contributions to xxx's IRA? in prior years, I put in the 2023 megabackdoor roth value under xxx's Total Basis as of December 31, 2023. This value is from form 8606 or 1040 in 2023 tax returns 

This workaround worked wonders! Thanks a lot!