Please follow the below template, it will help us to help you!
I should be able to define a static DHCP using the GUI.
Red box at top of screen with this;
There was a problem applying your settings.
PHP error (2): fopen(/etc/dnsmasq.d/04-pihole-static-dhcp.conf): failed to open stream: No such file or directory in /var/www/html/admin/scripts/pi-hole/php/savesettings.php:68
Raspberry Pi 2B
16GB storage, of which 3.4GB used.