@Brittany, Thanks for posting in Q&A. From the error message, it seems the storage space is not enough to add another account. If the device is still accessible, please firstly check on the device side to see if the available enough storage on the affected iPads and see which used the most storage space:
Note: Non-Microsoft link, just for the reference.
However, if the device is frozen, you can follow the steps in the following link to see if it can help.
https://support.apple.com/en-us/HT212017
Note: Non-Microsoft link, just for the reference.
Please try the above suggestion and if there's any update, feel free to let us know.
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