Conditional Formatting keeps wanting me to type apostrophes

IF(logical_test, value_if_true, [value_if_false])

logical_test (required) - The condition you want to test.
value_if_true (required) - The value that you want returned if the result of logical_test is TRUE.
value_if_false (optional) - The value that you want returned if the result of logical_test is FALSE.

your logical_test is : ISNUMBER(06) = always TRUE
but you forgot about value_if_true and value_if_false so function IF() doesn't know what to show.

edit:
but if you want to check number or not number enough to use =ISNUMBER(A1) (cell address change suitable). Result will be TRUE/FALSE so it will enough for CF