Presumably you've checked that these plugins are compatible with your version of InDesign?
I also have the same question!
My desired add on claims to be compatible with the software installed on my machine (CC 2015), and I even attempted to download and install through extension manager. I browsed the forums for solutions to installation fails and attempted to install without internet connection through extension manager as some posts indicated it could be a corporate firewall setting (I have no control over that).
With these results:
Does anyone have any thoughts?
You have to contact your plugin provider as it detects the last version, which is 126.96.36.199, wrongly.
Your downloaded add-on was added to the store in November 2015.
At that time InDesign CC 2015.4 did not exist. Could be that it's not compatible with the latest version.
As Willi said: Contact the developers.
Btw.: Their URL is relinking to a YouTube page.
And the plug-in's license agreement terms state that it is an Evaluation License.
Maybe that is not granted anymore?