Sorry, it should be
/ipv6/settings/set accept-router-advertisements=yes
If it doesn’t allow you to unset prefix-length, then set it to 64.
You can omit requesting address … it’s not always needed and some DHCP servers might have problems providing both in response to single request.
Are you using winbox4? I’m not closely following its development, but it might still have some teething problems here and there. So you can try using webfig (web UI) or CLI to verify that DHCPv6 client settings are indeed as intended.