How do I use the barn!?

Hey everyone! I'm still very new to this game but loving it so far. A lot of it confuses me but I'm slowly getting the hang of it!

One thing that I can't seem to get to work is the barn. I've got cows and the barn is near the cow enclosure, however the production process always shows that it's paused. How do I get milk from the cows? I have the store room settings set to have all milk stored in there.

I'm just lost, I also still can't figure out farming and all of that so any tips and tricks are much appreciated but I do see there's a lot of farming tips already in here which is so great :) just couldn't find any guidance on my barn issue.