Please read the all documentation if this doc is not working. The documentation linked below step by step

https://pub.dev/packages/msix

package config

pubspec.yaml

msix_config:
  publisher: CN=Blackshadow
  display_name: Blackshadow
  publisher_display_name: Blackshadow
  identity_name: MyCompany.MySuite.MyApp
  msix_version: 1.0.0.0
  logo_path: G:\Windows-Installer-with-NSIX-and-Certificate\assets\utils\logo.png
  certificate_path: G:\Windows-Installer-with-NSIX-and-Certificate\assets\utils\cert.pfx
  certificate_password: 1234 

First step

https://docs.microsoft.com/en-us/windows/msix/package/create-certificate-package-signing

PowerShell

New-SelfSignedCertificate -Type Custom -Subject "CN=Contoso Software, O=Contoso Corporation, C=US" -KeyUsage DigitalSignature -FriendlyName "Your friendly name goes here" -CertStoreLocation "Cert:\CurrentUser\My" -TextExtension @("2.5.29.37={text}1.3.6.1.5.5.7.3.3", "2.5.29.19={text}")

PowerShell

Set-Location Cert:\CurrentUser\My
Get-ChildItem | Format-Table Subject, FriendlyName, Thumbprint

predefined certificate path

G:\Windows-Installer-with-NSIX-and-Certificate\assets\utils\cert.pfx

Thumbprint

4E846D68B6C60EB6B86D5DEFF34EC3739AA03508

Export a certificate. Password usage

doc have to replace the thumbprint & certificate path

$password = ConvertTo-SecureString -String <Your Password> -Force -AsPlainText 
Export-PfxCertificate -cert "Cert:\CurrentUser\My\<Certificate Thumbprint>" -FilePath < FilePath >.pfx -Password $password

ready to run

$password = ConvertTo-SecureString -String 1234 -Force -AsPlainText 
Export-PfxCertificate -cert "Cert:\CurrentUser\My\4E846D68B6C60EB6B86D5DEFF34EC3739AA03508" -FilePath G:\Windows-Installer-with-NSIX-and-Certificate\assets\utils\cert.pfx -Password $password

Second step

https://community.flexera.com/t5/InstallShield-Knowledge-Base/How-to-add-certificates-to-the-Trusted-Root-Certification/ta-p/4118

1. Click Start, click Start Search, type mmc, and then press ENTER.

2. On the File menu, click Add/Remove Snap-in.

3. Under Available snap-ins, click Certificates,and then click Add.

4. Under This snap-in will always manage certificates for, click Computer account, and then click Next.

5. Click Local computer, and click Finish.

6. If you have no more snap-ins to add to the console, click OK.

7. In the console tree, double-click Certificates (Local Computer). double-click Certificates.

8. Right-click the Trusted Root Certification Authorities store - select All Tasks.

9. Click Import to import the certificates and follow the steps in the Certificate Import Wizard.

10 Browse and navigate to the .pfx file. and next

11 Password 1234 and next => next => Finish => OK

Third step

flutter clean
flutter build windows --release
flutter pub run msix:create

### msix created: build\windows\runner\Release\windows_installer_msix_dertificate.msix

ERROR

If you follow my process.

I hope this error will not face the: This app package is not signed with a trusted certificate.