How do I file last year's taxes? I missed the deadline so I am trying to do both but it is only allowing 2024?

Sorry you have to do each year separately.  Online is only good for 2024 now.

To do a prior year return you have to buy the Desktop program here,
https://turbotax.intuit.com/personal-taxes/past-years-products
You will need a full Windows or Mac to install it on.

If you have a simple return and want to file for free you can fill out the forms by hand. Here are some basic forms…..

Here is the IRS 2023 Form 1040
https://www.irs.gov/pub/irs-prior/f1040--2023.pdf

or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2023.pdf

And 2023 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2023.pdf

2023 Tax and EIC Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2023.pdf

Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2023.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2023.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2023.pdf

Then when you do file 2024 enter 0 for the 2023 AGI since you are filing 2023 late.

You have to print and mail prior year returns. When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return. Don’t forget state.

Use this TurboTax Web Page to File your 2023 taxes