Dual classing

I have created a human Ranger. She is now 4th level, I would like to dual class her to a cleric. Problem is that the dual class button is dimmed and I can't press it. Is it too soon to do this or what? I don't want to start over for the third time, so can anyone tell me what might be wrong?
Thanks

[Erekose]

you can't dual class rangers, paladins or bards. A bit late to tell you, sorry.

I dual classed my ranger to druid. Maybe the problem is not having either the 17 req. in wisdom for cleric or the 15 minimums in ranger prime reqs? (strength & wisdom, I believe). Note, the minimums have to be your natural stats, not boosted through belts, gauntlets, etc. Paladins and bards can't dual class.

Two other things: you only need to be level 2 to dual class, and rangers can only dual to cleric. A weird benefit of ranger/cleric is that you get to use both cleric and druid spells, but of course, you can only use cleric friendly weapons. Another strange thing about ranger/cleric, though, is that because you can use druid spells, you can use the "flameblade" spell, very useful for those pesky trolls. So if you're planning ahead, you can put some proficiency points in long swords while ranger, even though clerics can't use anything but the flameblad.