Lookup Formula Assistance
 

Good morning
I have posted other requests and been given help so hope someone can help me decipher this question.

The goal I am trying to accomplish is having a list produce results based on an Index formula's results.

I have DIV and PLAN feeding the BILLTO and INVOICE LEVEL cells respectively. What I am attempting to add is that when the INVOICE LEVEL = PLAN, the PLAN TO BILL cell has a drop down which allows the user to select the plan they need from the CLIENT PLANS worksheet. I would like the same to happen if the INVOICE LEVEL = AGROUP, the group drop down to give a list of Groups from the Client Groups worksheet.

I attempted to use the following formula, but am told the formula is invalid to make a list.
=if(b4="PLAN",(vlookup(b3,PlanList,2,false)))
The formula DOES work to bring one result when in the actual cell, but I need it to produce a drop down list.

Please let me know if there is a way to get this to work for making a list, and if anything else is needed, please let me know.

Thank you very much for any assistance that can be provided.