Making ringtones using iMac

Hi, I used to make ringtones for my iPhone using my iMac. I am now running Catalina 10.14.7 and am having a problem.

I have been right clicking on the tune I want to use, get info, changing the timing of the piece on the track I want to use and clicking OK.

Then when I want to convert to an AAC version it is not giving me that option. I am only able to choose Convert to WAV.

Any help appreciated please.