What to do if I am on my last month of isotretinoin (acutane) and I want to start to take beta carotene, can I take them when I'm on acutane?

I wouldn't. There's no health benefits to beta carotene in this situation. Both are of course related to vitamin A and there are a few unknowns. Congratulations on completing your course of isotretinoin -- it's not for the faint-hearted and chances are it's been of huge benefit to you. Best wishes.