Hello Juan9999,
This looks like a cosmetic issue with the webui .
The steps you took look correct and I was able to see the same behaviour in my lab. The issue does not occur when the zone selected is different from the zone for the previous interface. I selected the zone as untrust and it allowed me to add the interface. I was able to edit the interface later and change it back to trust.
We do not need to use the VLAN interface but the sub-interface option only. Regarding DHCP server, you can apply the config after you enter the correct VLAN and thus change the IP to obtain via DHCP.
Regards
Vatsa