Sorry for the delay in response. It looks like the package name in the original package xml file was incorrect, which is what is causing this problem.
To fix the uninstall, you'll need to do the following:
1. In your database #__extensions table, please find the row for the Sendy package and change the 'element' field to "pkg_sendy"
2. In administrator/manifests/packages/pkg_sendy.xml file, please replace
<packagename>Sendy Integration for Joomla</packagename>
with
<packagename>sendy</packagename>
3. Go back into the Extension Manager (possibly need to Refresh Cache for the sendy package)
4. Select Sendy package.
5. Click Uninstall
This should fix your problem. Sorry for the inconvenience and let us know if you still have problems after performing the above steps.
-Melissa