See this TurboTax support FAQ for error code 603 - https://ttlc.intuit.com/community/tax-topics/help/error-code-603-all-the-available-activations-for-that-code-has-been-used/01/1532406
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After filing 2021 tax, I found an error in 2020 tax filing and needed to amend 2020 tax. My old laptop which had 2020 TurboTax deluxe was broken and I purchased a new laptop in March 2022. I had all my software cloned to my new laptop. I could open 2020 TurboTax on my new laptop and entered the license code as requested by the software, it gave me an error code "603". I haven't activated the 2020 TurboTax since I last filed my tax return and I don't think I have reached the activation limits. Reading other responses regarding this issue, I have the impression that the only solution is to "buy the software again".
Please see this Turbo Tax help article.
Error code 603: All the available activations for that code has been used.
https://ttlc.intuit.com/community/tax-topics/help/error-code-603-all-the-available-activations-for-that-code-has-been-used/00/1532406