What Cast Spell button?
The Cast Spell button is on the combat frame, during the encounter phase. If you end a turn in a clearing with no enemies (other characters, monsters, natives), and your “Daily Combat” character option is disabled, then the combat frame wont even pop up, and you never have a chance to cast your spell. This can be a problem if (for example) you are the White Knight, and you are trying to cast “Make Whole” to fix your armor, and heal your wounds. If you are a spellcaster of any level (even the White Knight), I recommend leaving the “Daily Combat” checkbox checked, so that the combat frame is available every day.