I failed to take the hctc for previous years. I bought my healthcare plan on the Marketplace at full cost. How do I amend my tax returns to receive tax credit?

Before attempting to amend your tax returns make sure that you were eligible for the HCTC.

The Health Coverage Tax Credit is not available. The credit was not extended and can't be claimed for coverage months beginning after 2021.

Read the instructions for Form 8885 Health Coverage Tax Credit - https://www.irs.gov/pub/irs-pdf/i8885.pdf

See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-already-filed/L4VjJ9BA2_US_en_US?uid=lcgc34gd

I don't think you mean HCTC. Health insurance from the Marketplace was not eligible for the HCTC. You are probably thinking of the Premium Tax Credit (PTC), which is the "subsidy" for Marketplace insurance. If you did not claim the PTC on the tax returns that you filed, when you amend your tax returns enter the Form 1095-A that you got from the Marketplace. TurboTax will then fill out Form 8962 and calculate the PTC. Go to the link that DoninGA provided for instructions to amend your tax returns.