A "tablix" is a data region that can be configured as a table, a list, or a matrix. I have been making the assumption that your data expressions were inside the context of some kind of tablix, since that has been a requirement since SSRS 2008 R2.

Is this a SSRS 2008 R2 report design? If this was created before SSRS started to use tablix regions, then it was also before the LookUp function was added, which was 2008 R2.