Google Ad Manager is a great platform to serve ads, but it has a few bugs that Google still has not addressed. Personally I was able to add New Order, and New Ad Network Order, to create some orders, but then I started seeing the error:

"A company with this name already exists. Choose a unique company name."

This was a new company that I hadn’t used before, yet I kept getting this error after I tried to create a New Ad Nework from the drop-down-menu, which had not happened before. I found many complaints on the Goolge Ad Manager chat forum from over a year ago regarding this problem, but no solution was offered by Google Support.
I could not find a solution anywhere, so I’ve documented a solution below.

Below is the error I kept seeing.

To workaround this problem click on Admin -> Companies -> + New Company to create a new company before you try to create a new order. Give the new company a name, choose the type, and save.

Now go back and create your new order. The company will now be listed in the drop-down list, which avoids the bug, and the order can be saved without any further problems.