How to generate a unique id based on a dropdown list

I have been trying to generate a unique id based on a drop down list ive created in my spreadsheet. The dropdown list gives 4 choices of media format (audio recordings, film and video, documents and images). I want a unique id given to each row entry, displayed in this example format: AR-000001

I have created a table in a Lists sheet that has the media formats in the first column and the shortnames (ar, f&V, doc, and img) in the second column.

I have tried using the following formula (as suggested in the thread "generate a unique id" which seemed to be exactly what i needed):

IFERROR(VLOOKUP(A3,TABLE3,2,0)&TEXT(COUNTIF(A$3:A3,A3),"-000000"),"")

what am i doing wrong? these functions are a bit new to me so i am obviously missing something somewhere!

MUCH thanks in advance!

I suggest you shorten your table - you don't need to define it as having 1 million+ rows, as it will automatically grow when you add data. Delete all rows below, say, row 10, then you can put this formula in cell G3:

=IFERROR(VLOOKUP(A3,Table3,2,0)&TEXT(COUNTIF(A$3:A3,A3),"-000000"),"")

It should automatically fill down with unique IDs.

Hope this helps.

Pete