Add Letter Into Number

As HansVogelaar​ has already said, you can't have a fixed value and a formula in the cell at the same time. He gave you one solution.

Another solution would be to have the "raw numbers" (i.e., the as yet unused incident numbers) off to the side. Then, when an incident occurs with Luke Skywalker or Minnie Mouse, you create the "lettered incident number" in your column D.

See the attached as an example. Column D contains a formula, nested in an IF such that the resulting full incident # only gets completed when all the other data are entered. It's just a proof of concept. I assumed a different letter each time, for example; if your reality calls for a certain letter based on conditions you know, you could write another formula to generate that letter.