Hello Lina Maes,
DISM edition conversion typically fails because the target edition is not a supported conversion path for the installed evaluation image. Before converting, run DISM /Online /Get-TargetEditions; if the desired edition is not listed, the conversion is unsupported and a clean installation of the target edition is required. Microsoft specifically states that an Evaluation Datacenter installation can only convert to Retail Datacenter, not Retail Standard, and Desktop Experience cannot be converted to Server Core beginning with Windows Server 2016.
Another common cause is that the server is an Active Directory Domain Controller. Microsoft does not support converting an evaluation domain controller to a retail edition; the recommended approach is to deploy a licensed server, migrate AD DS roles, and retire the evaluation server.
If DISM returns errors such as 1168 or "target edition not found," verify that the product key matches the exact edition and that the edition appears in Get-TargetEditions. Also review C:\Windows\Logs\DISM\dism.log for the specific failure during the edition servicing phase.
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HL.