You need to find a newer Mac or use a Windows computer. Maybe you can use a friend's or family's computer?
If you can't install 2024 program you need to get a refund then you can use the Online version. You can't apply the desktop purchase to the Online version. The Online version you have to use is based on what you need to enter. So I would start in Deluxe and if you try to enter something that Deluxe doesn't have you will have to upgrade to Premium. You don't need to pay until you want to file or print. And state is a separate fee.
Sign in or start a new account here,
https://turbotax.intuit.com/personal-taxes/online
If you switch to the Online version you can transfer your 2023 return done in the Desktop program to the Online version. Post back next year if you need help transferring.
How to transfer into Turbo Tax Online, including from a pdf file or other software or from the Desktop program
https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-y...
If you used the Desktop program before you may notice several differences and not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2024 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.
