If I go back, it asks me start from the scratch again. Do I have to fill all the details and pay again?
You cannot transfer the 2020 tax file AFTER you enter new 2021 data. You have to do that 2020 tax file transfer first before you enter any new 2021 data at all. Trying to do that after you entered 2021 information wiped out what you had entered.
How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883
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Yes, you have to start over. Uploading a prior year data file must be the first thing you do, if you plan to transfer your file.
You cannot transfer the 2020 tax file AFTER you enter new 2021 data. You have to do that 2020 tax file transfer first before you enter any new 2021 data at all. Trying to do that after you entered 2021 information wiped out what you had entered.
How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883