Welcome to the forum.
Please when you have issues with blenderseed then add as much information as possible. Especially which version of the plugin you tried to install. The blender plugin is in constant development with frequent changes and it helps us to track problems. The latest release version can always be downloaded from https://github.com/appleseedhq/blenderseed/releases
The first error message seems to be some incompatibility issue. There is an Assertion Error after registration of the Radeon Pro Render plugin.
Maybe you make a clean install of Blender without additional plugins and then try to install blenderseed and see if the problem persists. If it’s a genuine bug, the main developer, @Jonathan_Dent will be thankful for the report.
I second Mango’s suggestion to disable ProRender. I’ve never seen that error before.