Finally, and specially if this is the first time you define Versions for a project, you need to make sure that the Affects Version/s and Fix Version/s fields are part of the screen to Edit/View issues because they are not included by default:
Project settings page, locate and click on the Screens option....use this screen scheme frame, look up for the Edit Issue operation and click on the link next to it.Affects Version/s and Fix Version/s fields to see whether they are listed. If they are, you can stop here; otherwise continue with the next step.Field name combo is visible.Field name combo and select the Affects Version/s field.Add button.Fix Version/s field.