Can't Deliver Cargo - Mining Mission

I'm a newbie. I've been playing for about a week. I spent my first few days training and shooting pirates. Yesterday, I started to mine for the first time. I know this is probably asked 1,000 times and I read all the Google articles and watched YouTube. My problem is not quite the same. I took on a mining mission. So, I went to go mine, got the material, came back and went...
132206 MISSIONS BOARD to my original mission. I pressed + at the bottom for the all the units requested. Lastly, I hit COMPLETE and it does nothing.
The image I have attached is my 2nd attempt and I only got half of the units but I was told I can turn in partial. won't let me do that either.

Yes, I read the original mission. It said for me to go mine for the mineral...not go deliver it or go trade for it. I did exactly what it asked. What I'm I doing wrong?
The slider is also a button. (I know, not intuitive.). You have to

1. Get the correct number on the slider.
2. Click the slider which is a button.
3. Click complete.

I guess they did it this way so you can do partial deliveries, the Complete button is to complete the whole mission once all the pieces are delivered.
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