Multisite, domain mapping plugin with subdirectories…cannot map domains

Hello!

I have WordPress Multisite installed and I am trying to use the domain mapping plugin. I have a dedicated IP. I'm using subdirectories. When I create a new site, I create the sites's domain A record to point to my static IP. When I go to map the domain to the subsite, I get the error (attached). I'm at a loss as to what I'm doing wrong…I've spent hours reading old posts, etc. and I just can't figure it out! I'm hoping you can help.