There are three main reasons why a package search would not be successful:
1. That account doesn't have a license or account's license doesn't cover that version
2. The package version doesn't exist
Go to the UI for Xamarin downloads page
and check that your account has access to that version:
If you do not see that version, you'll need to change your credentials to the account that did have that license or you contact Sales
and ask they "backdate" your UI for Xamarin license.
You shouldn't be using clear text credentials in the nuget.config. You instead use the environment variable that AppCenter/DevOps provides. You can review the nuget.config I use in one of my demos: https://github.com/LanceMcCarthy/ComicVine
Here's what the Environment variable section looks like in AppCenter:
Lance | Technical Support Engineer, Principal
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