Why can't I use my great prophet to enhance my religion?

Solution 1:

To enhance a religion, a great prophet needs to:

• Be in the holy city of your religion- not the capital if these are different cities.
• Have all its spread religion uses remaining. If you use him to spread your religion even once, you can't use him to enhance your religion.
• Be of your religion. Are you certain the city you bought him in was of your religion at the time?
• Be of an unenhanced religion. You cannot enhance a religion twice.

Make sure all of these are true, especially the second one.

Solution 2:

You have to put the Great Prophet in the holy city of that religion (the city that your first Great Prophet founded it in). Then, the option should appear at the top of his command list.