Cannot resize Macintosh HD using (supposedly) available free space
This worked for me:

On the left is a list of drives your Macbook or iMac can see.  Select the drive from this left menu that has the partitions you want to combine.

In the top of the window will be five tabs/buttons.  Choose the middle one called “partition”.

Select the partition from the big block on the left that you want to remove.  With the partition selected hit the small “-” at the bottom left corner to delete the partition.  This small “-” is near the “+”.  This will bring up a message, okay that and it will delete the extra partition and leave empty space.

Now we need to fill that empty space with your main partition.  Still on this partition screen, you will see your main partition block has three small lines in the bottom corner.  Like the corner of a piece of paper when you are turning a page.  Click on here and you can drag the box size.

Drag the box size until it fills the space completely.  Hit apply and it will then make a big block, single partition