Hello @Шатравка Даниил ,
Welcome to Microsoft Q&A forum.
Could you try to check if this index(package source) has been set correctly in the nuget.config file(~/.nuget/packages)? I also suggest you back it(nuget.config file) up and then remove it(or create a new basic one) and run your apps again.
Besides, if you use any proxy, you may need to add related settings into the nuget.config file.
Best Regards,
Tianyu
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