Per IRS Publication 544, in order to have a capital loss (or gain), you must sell or exchange a capital asset (such as cryptocurrency).
From your description of events, it doesn't appear that you ever had ownership of a capital asset. Therefore it appears that you don't have a basis for claiming a capital loss.
https://www.irs.gov/pub/irs-pdf/p544.pdf
Instead, you have a theft loss, as described by @xmasbaby0.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
