力不足 means “someone doesn’t have enough abilities for a task.”
役不足 originally meant "The task is too insignificant for someone who deserves a better job," but now many Japanese people use 役不足 as “someone doesn’t have enough abilities for a task.”
役者不足 is a coined word, which you cannot find in dictionaries, and I agree with Chrispayneable, it is not a common word.
There is no absolute interpretation of this word, but many people use 役者不足 as “someone doesn’t have enough abilities for a task.”
では(an auxiliary verbだ+は) generally indicates A is the premise of B.
あなたでは役者不足(だ) = As far as you take this job, it would be too heavy (for you). (There might be a better person)
あなたは役者不足(だ)= You are not good enough for this job.
These two are fundamentally the same, maybe the second あなたは役者不足だ is a bit more direct.