Hi @Kevin Dule

I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.

Issue:

when you attempt to log in from the first tenant, you encounter the following error: "AADB2C99002: User does not exist. Please sign up before you can sign in."

Solution:

To fix this issue you have prepared a script using Microsoft Graph. Users from Tenant A should be created also in Tenant B2C. So, you have defined these identities for these users

If you have any other questions or are still running into more issues, please let me know. Thank you again for your time and patience throughout this issue.

Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.

Hello,

I am attaching Powershell Script for creating users from another Tenant. This script will solve the error "AADB2C99002: User does not exist. Please sign up before you can sign in."

$Tenant1ID = "your-tenant-id"
$B2CTenantID = "your-b2c-tenant-id"
$listOfUsers = @("user1@example.com")

Write-host "Connecting to Tenant 1: " -ForegroundColor Yellow -NoNewline
Connect-MgGraph -TenantId $Tenant1ID -Scopes user.read.all -NoWelcome -ErrorAction SilentlyContinue

if((Get-MgContext).TenantId -eq $Tenant1ID) {
    Write-host "Done.`r`n" -ForegroundColor Green
    $users = @()
    Write-host "Retrieving users from Tenant 1:" -ForegroundColor Yellow
    foreach($user in $listOfUsers) {
        $tenantUser = Get-MgUser -UserId $user -Property id,displayname,surname,givenname,mail -ErrorAction SilentlyContinue
        if($tenantUser) {
            Write-Host "`tUser retrieved: $user" -ForegroundColor Green
            $users += @($tenantUser)
        }
        else {
            Write-Host "`tUser not found: $user" -ForegroundColor Red
        }
    }
    
    Write-host "`r`n`r`nConnecting to B2C tenant: " -ForegroundColor Yellow -NoNewline
    Connect-MgGraph -TenantId $B2CTenantID -Scopes user.readwrite.all -NoWelcome -ErrorAction SilentlyContinue

    if((Get-MgContext).TenantId -eq $B2CTenantID) {
        Write-host "Done.`r`n" -ForegroundColor Green
        Write-host "Provisioning users:" -ForegroundColor Yellow
        
        foreach($user in $users) {
            Write-Host "`tWorking on $($user.displayName)[$($user.id)]: " -ForegroundColor Yellow -NoNewline
            $params = @{
                displayName = $user.displayName
                identities = @(
                    @{
                        signInType = "userPrincipalName"
                        issuer = "your-b2c-domain.onmicrosoft.com"
                        issuerAssignedId = "cpim_$($user.id)@your-b2c-domain.onmicrosoft.com"
                    }
                    @{
                        signInType = "federated"
                        issuer = "https://login.microsoftonline.com/$($Tenant1ID)/v2.0"
                        issuerAssignedId = $user.id
                    }
                )
                passwordProfile = @{
                    forceChangePasswordNextSignIn = $false
                }
                passwordPolicies = "DisablePasswordExpiration"
                AccountEnabled = $false
            }

            if($user.GivenName) {
                $params.Add("GivenName", $user.GivenName)
            }
            if($user.Surname) {
                $params.Add("Surname", $user.Surname)
            }
            
            $newUser = New-MgUser -BodyParameter $params
            if($newUser) {
                Write-host "Done." -ForegroundColor Green
            }
            else {
                Write-host "Failed." -ForegroundColor Red
            }
        }
    }
    else {
        Write-Host "Unable to connect to B2C tenant." -ForegroundColor Red
    }
}
else {
    Write-Host "Unable to connect to Tenant 1." -ForegroundColor Red
}