No, once you've accepted a voucher, you voluntarily waived your right to reimbursement (though, maybe you could try to argue that you were tricked into taking the voucher, but once again, that's matter for the courts and not a straightforward case).

But: if your flight was canceled for any reason (incl. Covid-19), you have the right to be reimbursed in cash – not by voucher – within 7 days, because the relevant EU legislation (EC261/2004) is valid and takes precedent over national law for as long as it exists in this form (and once again, the Commission cannot and will not change this). If the airline does not comply, you can turn to the National Enforcement Body for arbitration and/or go to court. Of course, if you're not prepared to do so, you're gonna have to play by the rules of the airline, whether they are following the law or not.