Region for Main page content block field is required Error?
This post helped me get closer, but I thought I would provide a more concise answer for those following the same path.  

If you are looking to remove all blocks from Drupal 7, like you would want to do if you were placing blocks by using Context, you want to make sure that core's "Main page content" is the last block removed.

Steps:


Select "- None -" for all other blocks, then click Save.
Then change the "Main page content" block's region to "- Select -" and
click Save.