If the tube works fine with the starter out, we know the tube is fine. Don't just give it a very quick check, let it run a good 5 mins like that.
If adding the starter it then keeps flashing, we know the fault is the starter. It could be faulty or simply the wrong type. Most are 4-80 or 85w, but some are 20w max, some are 120v or series, some are 100 or 125w for 8' tubes. Check you have a suitable starter.
NT