Since dev-branch version 1.43.129986 you will always get the message "No player was selected!" if you try to start a self created multiplayer mission (playable units available).
That message will only disappear, if you created a player unit. That was normally not necessary as long as there are available playable units in your multiplayer missions.
Tested with the latest two dev-branches (1.43.129986 and 1.43.130012)
@Killzone_Kid: Thanks for the work around.
I hope that small issue will be fixed soon.
Hello and thank you for your feedback.
I apologize this is not a issue. As Killzone suggested use the description.ext as work around for now or place a Player unit in the editor. It will behave just like normal playable unit.
Disregard my last post. It is an issue and has been scheduled for a fix. Thank you for your feedback and patience.