If you had big amount of Bitcoin and bought it from exchanges with KYC verification, you should ask your lawyer who's expert on cryptocurrency tax.
Don't rely too much on advice from stranger.
After reading all the comments I found this as the genuine and honest reply to OP.
What I believe is you still have to pay tax because you were entitled to pay tax when you bought the coins so no matter if you lost them or not you are still required to pay tax even so you tell them that you have lost.
It all depends on where you live and how you obtained the BTC.
Where I'm at if I'd just bought it then no. If I got it by earning it and trading if it's above the limits they give then I'm still liable for the tax on those earnings even if I have no way of paying it.
Look at these people who did a million shitcoin trades, wound up with nothing and didn't realise every trade was taxable.
Still, I wouldn't bother mentioning it.
The op has not provided any kind of information to us whether he brought those coins or he earned it via some services he may have provided, and till he doesn’t clarify this how are we supposed to guide him about his tax liability?. @gentlemand how does the taxman find about these trades if the person doesn’t disclose it?, or do taxmen get inputs from the exchanges regarding these transaction can you clarify this part?.
If the OP is saying that he still have to pay tax... then it makes us clear that he must've used some exchange or some way where he has provided the information like KYC etc otherwise he wont bother to ask.
P.S why I am unable to give merit? It says 0
don't worry reply me in inbox if someone knows why I have 0 to keep the thread cleanRegards