Alternatively said:
1. Is the class actually available in your game?
At character creation look to see what classes you have. Official expansions MOTB (Mask of the Betrayer) and SOZ (Storm of Zehir) offer additional classes (and feats, etc..).
2. You must have at least one *base* class.
These classes have no *prerequisites* and can incur multi-class penalties.
3. You can have *up* to 4 classes *total*.
In any combination of *base* and *prestige*. EX #1 - 1 base class, 3 prestige classes. EX. #2 - 2 base classes, 2 prestige classes. EX. #3 - 3 base classes, 1 prestige class. You do not have to have more than one class however.
4. Only *prestige* classes have prerequisites and have no multi-class penalties.
You must meet the conditions of those prerequisites before that class can be taken.
5. Prerequisites for prestige classes often have their own prerequisites.
Prerequisites that include feats often require their own prerequisites. EX. a Frenzied Berserker's feat prerequisites include: Cleave, Great Cleave, and Power Attack. To be able to select Power Attack however your character must have a 13+ non-enhanced Strength. To be able to select Cleave you must first have Power Attack. To be able to select Great Cleave you must first have Cleave.
On rare occasions certain prerequisites are not met when they would appear to be. EX. The Arcane Trickster's requirement of 2d6 of Sneak Attack damage is NOT met by 2d6 of an Assassins Death Attack. This leads to this suggestion:
6. Any build you envision should first be tested in a "leveler" program so that you progress in the actual game in the manner you intended.
Vordan's Hero Creator is a good "leveler" program:
Vordan's Hero Creator -- Neverwinter Nights 2 Vault
Note the Installation Instructions:
"INSTALLATION:
(1) Move the file "Vordan's Hero Creator.mod" into your My Documents/Neverwinter Nights 2/modules folder.
(2) Move the file "vhc.hak" into your My Documents/Neverwinter Nights 2/hak folder."
Links:
Classes:
Classes - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Base Classes:
Base Class - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Multi-Class (base class) Penalty:
Multiclass penalty - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Racial Favored Classes:
Favored class - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Prestige Classes:
Prestige class - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
BAB:
Base attack bonus - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
Note that a BAB prerequisite is NOT your modified attack number. On your character sheet you'll see it BELOW your attack numbers. Note that mult-classing can alter your BAB progression.