I am a bit confused by the "The following rights cannot be granted because they conflict with the reseller plan" message you're getting. I am not sure why you would get this error if there are no reseller plans.
However double check this:
1) go Service Plan and select the service plan used by your domain(s)
2) go to the Permissions tab and check if the "PHP version and handler management" option is selected. If not, select the option and hit the save button.
View attachment 20046
If it is already selected, then go to subscriptions and click on the domain you want to change the PHP version for.
Then on the right side menu click the Customize link.
View attachment 20047
(If there is no right side menu visible on your domain, click the small menu icon
View attachment 20048 on the top side of the page first to open the right side menu.)
Then click on the Permissions tab and check if the "PHP version and handler management" option is selected. If not, select the option and hit the save button.