The AuditDB.LastProcessedDate is in the datamodel.
But in which tables? And in which row? How should Qlik Sense know which table it should look in and which record it should get the value from? The same field can exist in many tables and a field in one table can have many values.
That's why you have to be explicit and use the peek function to get the value you want from the table:
LET vMyDate = peek('[AuditDB.LastProcessedDate]', 83949, 'MyAuditTable');
Replace MyAudit table with the name of the table from which the field value should be retrieved and replace 83949 with the row number in that table that contains the value. When using peek 0 means the first row and -1 means the last row.