In ERA for RMC, when you open the spell casting dialog, all spells known to the caster are shown. There is no restriction that limits what you can choose.
After the spell is selected, penalties are shown to the right of the roll (bottom left of the dialog). There's a magnifying glass icon that opens a detail on the modifiers.
If the caster has prepared the spell you should enter them in the "Preparation Rounds" field and that will adjust the penalties accordingly.
After casting the spell, in case it succeeds, you should roll for the attack by using the attack dialog and choosing the directed spell from the lists of OBs.
Any result of N/A in a dialog after entering the roll might indicate an issue with the system configuration. Some book packages might not be installed, or a combat skill might not be indicating the correct attack table.
If you'd like to share screenshots of the situation you are observing, please send them to the support mail account: era–support.voriigkye@recursor.net