How can I fix this?
Before I tried any of the solutions, I would simply, download the Visual Studio 2019 Community installer again. If that does not work try each solution in order.
Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options -> System cryptography
Disable: Use FIPS compliant algorithms for encryption, hashing, and signing
However, this typically happens when the certificates used to sign the installer, are not trusted. Be sure you are installing the current Visual Studio 2019 build. You also must verify if you have all available Windows 7 updates installed.
Source: When the setup start, I receive the message "The package manifest failed signature validation"
If the certificates are not trusted, mount the Visual Studio 2019 ISO, and import the certificates in the appropriate folder. This problem was fixed more then 2 years ago, so if your system is current, and you are using the current Visual Studio 2019 installer, you will have to import the certificates yourself.
You should also update your system's certificates. You will need the rootsupd in order to acomplish that.
- Extract the contents
rootsupd.exe /c /t:C:\temp\newroot
- From within the directory, in an elevated command prompt, issue the following commands.
updroots.exe -l roots.sst
updroots.exe -d delroots.sst
Source: VS2017 installer - the package manifest failed signature validation