iOS option is disabled in Version or platform (iTunes Connect)

I had the same issue. I had to reject an app that was in "Waiting for Review" status since we had to update a bug that we found after we submitted the previous build. Once we did that, we were unable to add a new version and the ios option was disabled. To fix this:

1. I went to the same version that showed "Rejected by Developer",

2. Scrolled down to the build version,

3. Clicked the minus (-) icon next to the previous build to remove that,

4. Added the new build and

5. Saved and Submitted for review.

The version increment automatically updated after step 5 above based on the selected build.

I found the solution. In my case I wanted to update the app to a newer version which wasn't listed in the "+Version or Platform" menu. The possible reason is that a previous build failed, with the status of "Prepare for Submission".

1. Select the build which is listed as "Prepare for submission" under "iOS App".
2. Scroll down till you see the build for that submission.
3. Delete the old build and select a new correct build.
4. Now scroll down more and change the version of your submission in the textfield to the build number of your correct build.

After doing that I was able to submit it to the App Store.