Add IsNull() or Coalesce() around the transcription credits, like this:

select distinct p.first_name

,p.last_name

,'class level' = case when a.credits + t.earned_credits between 1 and 32 and a.program ='ft'then 'F1'

when a.credits + isnull(t.earned_credits, 0) between 33 and 61 and a.program ='ft' then 'F2'

when a.credits + isnull(t.earned_credits, 0) >= 62 and a.program ='ft'then 'F3'

when a.credits + isnull(t.earned_credits, 0) between 1 and 25 and a.program ='pt'then 'P1'

when a.credits + isnull(t.earned_credits, 0) between 26 and 47 and a.program ='pt'then 'P2'

when a.credits + isnull(t.earned_credits, 0) between 48 and 70 and a.program ='pt'then 'P3'

when a.credits + isnull(t.earned_credits, 0) >= 71 and a.program ='pt'then 'P4'

end

..........etc