In my case, the problem was that I was building on a, older virtual machine which was based on Win7.
I found this fix from https://github.com/NuGet/NuGetGallery/issues/8176#issuecomment-683923724 :
nuget.org started enforcing the use of TLS 1.2 (and dropped support
for TLS 1.1 and 1.0) earlier this year. Windows 7 has TLS 1.2 disabled
by default (check the DisabledByDefault
value under
HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client
in your registry). To enable the support, please make sure you have an update (*) installed and switch the support on:
reg add HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client" /v DisabledByDefault /t REG_DWORD /d 0 /f /reg:32
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client" /v DisabledByDefault /t REG_DWORD /d 0 /f /reg:64
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client" /v Enabled /t REG_DWORD /d 1 /f /reg:32
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client" /v Enabled /t REG_DWORD /d 1 /f /reg:64
The (*) update referred to was Microsoft kb3140245:
Update for Windows 7 (KB3140245)
I installed the update, rebooted (as requested by the update), added those registry keys, and then Nuget worked fine.