Community is here to help but remember the intention of Level 3 is to test each individual on their Anaplan skills as well on their analytical capabilities. Please do not share the formula on Level 3 related issues. There are other means to help as much as we can. Before taking Level 3 we had taken virtual oath of not sharing any formulae or any questions related to L3 with anyone.
Ensure that the format of the line item which informs your LOOKUP is either a time period or list. The dimension of the line item in the mapping must also be the same as the dimension of the data source.
Often users will try and build a LOOKUP whereby the line item is a TEXT format. TEXT formatted line items have no relevance in a formula requiring a mapping function.