My solve block says this variable is undefined

You have asked it to find 7 variables, but you only have 6 equations (in fact, I can't see I7 anywhere in your equations, so just delete it from the find list).

Alan

Additionally to Alans remark I noticed that you did not provide guess values for the variables (I1 .. I6) you are looking for. Thtas what the top section in the solve block is for. The variables R.. and E..  could be defined outside and above the solve block. But its no mistake to define them in the solve block (and it also would be no mistake to define the guess values outside and above).

And for future questions: Always attach your worksheet as well, as usually its quite hard to debug just a picture!