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On the screenshot, it shows the error when adding users and groups and per the details of your site, I can see that one site is a standard with 56 users and the other one is Free where it's possible to add only 10 licensed user.
I believe that this error might be related to the number of licensed users being migrated, so you need to deactivate the users and let only your account and other admin as active and import it again.
You can edit the current backup or deactivate the users and create a new backup. In order to set users as inactive from the backup, you need to edit entities.xml file.
For that, please find the information below and set the active="1" as active="0" for users that you want to let inactive.
<User id="10000" directoryId="1" userName="admin" lowerUserName="admin" active="1" createdDate="2019-10-30 19:10:53.538" updatedDate="2020-04-29 00:28:45.705" displayName="Angelica Luz" lowerDisplayName="angelica luz" emailAddress="email@example.com" lowerEmailAddress="firstname.lastname@example.org" deletedExternally="0" externalId="123456:1234a56b-c7de-8fg9-hij1-23456789k123" localServiceDeskUser="0"/>
After that, save the file and compress it again to import it on a Cloud site.
Please, give it a try and let us know how it goes.
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