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It looks in all repositories, but only if the repositories work.
As the plugin central is not accessible, it fails.
Otherwise the build could behave differently / pull in different versions / ... depending on where you build.

how gradle understand that repository not work if he don't check it?

or gradle, regardless of what I specify in the `pluginManager.repository{}`, is looking for in the portalPluginRepository?

You specified both.
It looks in order.
So it looks in plugin central first.
That fails, so it stops.