Stock I owned was delisted in 2019, but listed on OTC market in 2020 at essentially no value. Can I claim a loss in 2019?

[DeanM15]

You can claim a loss for worthless securities. If it is listed on an exchange at some, even very small, value that would not be worthless.

The IRS uses the term "totally worthless" and includes stock that you abandon. To abandon a security, you must permanently surrender and relinquish all rights in the security and receive no consideration in exchange for it.

If the security is listed on the OTC, I would attempt to sell it. That would give you a clear path to take your loss. If you are unable to sell it, I would ask your broker to buy it. You would likely end up with no money and perhaps even owe fees for the sale. But again you have a clear path on how to take the loss on your tax return.