You can claim this as a loss on the sale/disposition of an investment, but only in the year when the investment became worthless (e.g. the year they filed bankruptcy).
See How do I deduct worthless stock? for instructions on how to make this entry.
It appears you would need to amend your 2019 return to do this. See How do I amend my 2019 return? for instructions.
what type of entity was the startup? if a partnership or S-Corp you should have been getting a k-1 each year which likely would report ordinary losses reducing your taxable income and hence basis. Only with a C-Corp would you be entitled to a capital loss in the year it went bankrupt. For a partnership or S-Corp it's likely the losses wiped out your basis so there is no capital loss to report.