I created a custom scenario in the singleplayer. I clicked on "Save and Close" or something like that but I can't choose this distribution system... Why?

you can't use custom scenario in Multiplayer, you need to unlock it by leveling up first. I guess that's why you can't save it- since the new update included this:
"When saving a template, the game now checks that the template is valid before saving. This fixes an issue where a template could be saved but not loaded (for example, if local deployments was enabled on a map with overlapping bonuses)"

Does this also affect Singleplayer? That I can't make use of it because I'm too low level? Which level is required to unlock it?

have to be lvl 46 :/

EDIT: but i can save them.

Edited 12/30/2013 19:23:14

I can use custom scenarios in singleplayer. You should be able to use custom scenario for singleplayer game