Can I drink coffee while takin effexor (venlafaxine)?

Yes. There's no special restriction about drinking coffee while taking Effexor (venlafaxine). Of course, the caffeine may make you anxious or jumpy, but this would be true with or without the medication. In addition, Effexor is sometimes experienced as mildly stimulating itself, so this could add to the caffeine effect.
OK, decaf better. Caffeine is a stimulant and Effexor (venlafaxine) is a CNS -active agent so there is a possible interaction. However alcohol should be avoided. Another caveat: need to follow blood pressure with Effexor (venlafaxine): the use of Effexor is associated with dose-dependent, sustained increases in blood pressure, mostly in the range of 10 to 15 mm hg for diastolic blood pressure.