In bar 10, just select the LH a and select Ends voice in the properties window. You can tell they belong to the same voice because they’re the same colour.
In bar 17, you’ve created extra rests which don’t necessarily belong to a [useful] voice. You should start by deleting the first red rest. You’ll see that all three red rests disappear. If you then delete the second green rest, the other two will disappear as well. As a result, you don’t need to end any voices in bar 16.
I know, Dorico’s treatment of voices can be difficult to fathom sometimes and it really helps if you’re careful and methodical about entering the voices in the first place. I’m still getting used to it, but it’s a great system once you get it under your belt!