Hi Tee Gee,
this error has nothing todo with the type of the field but with the definition of the DB table. The FLTP fields of AUSP reference to the table T006. Within BRFplus there is a check (CL_FDT_DB_LOOKUP->CHECK_FIELD_MAPPING) that checks for the field mapping if the field has a reference table assigned to it and if this si the case if the reference table is different to the original table that the DB lookup is executed on. If this is the case the error message that you see is raised. This type of "configuration" is not supported in BRFplus.
How to overcome that? If you want to query the table and map that field you have to wrap the query into a static class method and use a procedure call expression to call that method.
Wrapping the table with a view (simple old-sytle DDIC or CDS won't do the job - BRFplus is too clever here ;-))
Hope that helps
Cheers
Christian
P.S. As a moderator I have to tell you that the picture you use is against the RoE of SCN. You have to use a real picture (and I do not think that you are Messi right 
)