This voucher is invalid or already redeemed. - I tried earlier, the key begins with the letter K and it does not work as a voucher. also it is not activated in electrum. I wrote to the mail indicated on the site, I am waiting for the third day, there is no answer
if it's a private key then it should come in pair with its bitcoin address
(at step 3 or signed receipt) can you check that address on block explorer?
if it is indeed unfunded
(never funded) then there was a glitch on ChipMixer system
and I'm pretty sure ChipMixer will rectify it either by funding or replacing it, just have to be patient