Hello @Jacek Wieliński ,
Please note that Sandboxes are no longer available for training or practice. You will need to use an Azure trial account or a pay-as-you-go subscription instead.
source: https://learn.microsoft.com/en-us/training/support/faq?pivots=sandbox#are-sandboxes-still-available
Therefore, I recommend not attempting to reactivate or rejoin the locked tenant from your previous sandbox. Instead, please use an Azure trial account or a pay-as-you-go subscription for your activities.
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