chloe22286,
You will have to use the bill of sale date to determine the year of purchase. It certainly appears that it was in December of 2018. The good news is that you can amend your 2018 return by April 2022 to include that major purchase sales tax on your 2018 tax return.
Please be aware that the sales tax alternative to state and local income tax is also an "itemized deduction" and so would be helpful only if it combined with other itemized deductions such as mortgage interest, charitable contributions, etc. would exceed your standard deduction ($12,000 in 2018). That's now a rather big hurdle, but certainly worth checking.