Push NuGet package: This package must be signed with a registered certificate

I'm trying to push my NuGet package with the commmand

dotnet nuget push *.nupkg --source https://api.nuget.org/v3/index.json --api-key *** --skip-duplicate

But I'm receiving the error:

error: Response status code does not indicate success: 400 (This package must be signed with a registered certificate.)

Is there any way to publish unsigned NuGet package? What are the requirements for unsigned packages?

I thought, it's not required to sign the package with a certificate. At least, I couldn't any information about this requirement in the official documentation.

What I remember, certificates for code signing are not free of charge, I would say they are quite expensive and it doesn't make sense for me to buy a certificate to publish one package once a year.