Can I amend property tax that I forgot to enter in 2021 if it is now 2023?

You can amend the 2021 tax return to include the property tax you paid in 2021---if you believe it will make a difference.   The amount of property tax and state and local tax you can deduct is capped at $10,000 (or $5000 if your file MFS)   and your itemized deductions must exceed your standard deduction in order for any of the deductions to have an effect.

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-a...

No. You would have to amend your 2021 return.