VLAN 0 (no such VLAN ID) should indicate "untagged" and typically generates that type of DHCP warning in Dashboard. So just out of curiosity, try this, click into that AP's page, click on the pencil next to LAN IP, and for the VLAN box, leave it blank, don't even put a "1" in there, just leave it blank and click save. Wait a few minutes and refresh the page, see if that does it. Should not have to reboot the AP, but try that too if it's not automatic.
To be sure, I just reproduced this exact issue in my home lab in just a few minutes by placing a VLAN 1 tag on an AP, got the warning, then blanked out the VLAN field, and warning goes away.
If the switchport is set as an access port then there is no native VLAN from the device point of view. From the switch port any traffic going into the switch will be put into the access VLAN, regardless of any VLAN tags. If you have a trunk port with a native VLAN then you need the VLAN to be set to that in the AP config.