Diagnosis
A text foreign key defines the relationship between a table and itstext table.
Usually the text table has the same key as the check table, with alanguage field extension. In the R/3 System, a language field isindicated by a domain of type 'LANG'.
Since in this case the foreign key table does not have a language fieldas key extension, you may not select "Key fields of a text table".

Procedure
Specify the type of foreign key field correctly.
The foreign key table in question might be a text table, but thelanguage field does not have type 'LANG'. In this case the languagefield must be reassigned to a domain of type 'LANG'.