Hi!
@Samuele Provvedi -Yes, the region has to be entered without spaces:
$Region = "WestEurope"
I had the very same issue and got the misleading error code "AZCM0026" until I specified the region as "WestEurope" and not "West Europe".
Here are the exact commands I used:
#Define the subscription where you want to register your server as Arc device
$Subscription = "0005dd00-d006-0a10-8000-000071300000"
#Define the resource group where you want to register your server as Arc device
$RG = "rg-weu-azshci"
#Define the region you will use to register your server as Arc device
$Region = "WestEurope"
#Define the tenant you will use to register your server as Arc device
$Tenant = "003b0000-9600-4300-9004-aa0024d00000"
Invoke-AzStackHciArcInitialization -SubscriptionID $Subscription -ResourceGroup $RG -TenantID $Tenant -Region $Region -Cloud "AzureCloud" -ArmAccessToken $ARMtoken -AccountID $id
AzureStack 23H2 with AzSHCI.ARCInstaller 1.2408.0.3053
Hope this helps!