Either you need a new certificate installed for this app package, or you need a new app package with trusted certificates

What it means

The package is signed, but Windows doesn't trust the certificate it was signed with. That's almost always a self-signed test certificate, made by Visual Studio, New-SelfSignedCertificate or a packaging tool, that was never added to this PC's trusted certificates.

People who install your app from the Microsoft Store never see this. Microsoft signs Store packages after certification, so you don't need a certificate to submit one. It only matters for packages you install yourself or ship outside the Store.

How to fix it

1. To trust a test certificate, import it into the Local Computer's Trusted People store from PowerShell run as administrator. Microsoft notes that App Installer checks the machine store, not the current user's:
   Copy

   $cert = (Get-AuthenticodeSignature -FilePath .\app.msix).SignerCertificate
   Export-Certificate -Cert $cert -FilePath .\app-cert.cer
   Import-Certificate -FilePath .\app-cert.cer -CertStoreLocation Cert:\LocalMachine\TrustedPeople

2. Without PowerShell: right-click the .msix, open Properties, then Digital Signatures. Select the signature, click Details, View Certificate and Install Certificate. Pick Local Machine and place it in Trusted People.
3. Don't put a self-signed certificate in Trusted Root Certification Authorities. Microsoft says that weakens the PC's security unless the certificate really is a root CA.
4. Open the package again. If it installs, your test setup is done. If signing failed instead, see publisher name must match. Our guide to installing an MSIX covers the other install errors.
5. For other people's PCs, sign with a certificate from a trusted CA or with Azure Artifact Signing (formerly Trusted Signing), which Windows trusts by default. For the Microsoft Store, skip signing and upload the package to Partner Center.