You can resolve this error by replacing the wimgapi.dll file. Please follow the steps below and then try to upgrade again:
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Open Windows Explorer.
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Locate the file: C:\Windows\System32\wimgapi.dll
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Select the wimgapi.dll file, then right-click the file and select Copy.
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Then locate and open the folder: C:\Windows10Upgrade
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Right-click in the open folder and select Paste.
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You should see a warning that there’s already a file with the same name at that location. Select Copy and Replace.