Error assigning DOI's - Dspace 6.3

[Julia Reis]

Dear all!

I hope you can help me. Our organization decided to use DOI instead of handles to the items. I followed the instructions  from Dspace Documentation but it gives me error still. We already have a prefix, username and password in DataCite. 
Can someone help me on how you were able to change this configuration and make it work?? Thanks!!

version: dspace 6.3 /jspui

This is the error message:

Update DOI doi:10.34960/24 for ITEM with ID 29a680b9-13fa-461f-afed-3a2b8bd088b6 failed:
UNKOWN

2021-04-10 14:56:03,978 ERROR org.dspace.identifier.doi.DOIOrganiser @ It wasn't possible to update this identifier:  doi:10.34960/24 Exceptions code:  UNKOWN
org.dspace.identifier.doi.DOIIdentifierException: Unable to reserve DOI doi:10.34960/24. Please inform your administrator or take a look  into the log files.
        at org.dspace.identifier.doi.DataCiteConnector.reserveDOI(DataCiteConnector.java:490)
        at org.dspace.identifier.doi.DataCiteConnector.updateMetadata(DataCiteConnector.java:582)
        at org.dspace.identifier.DOIIdentifierProvider.updateMetadataOnline(DOIIdentifierProvider.java:420)
        at org.dspace.identifier.doi.DOIOrganiser.update(DOIOrganiser.java:594)
        at org.dspace.identifier.doi.DOIOrganiser.runCLI(DOIOrganiser.java:240)
        at org.dspace.identifier.doi.DOIOrganiser.main(DOIOrganiser.java:71)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.dspace.app.launcher.ScriptLauncher.runOneCommand(ScriptLauncher.java:229)
        at org.dspace.app.launcher.ScriptLauncher.main(ScriptLauncher.java:81)
2021-04-10 14:56:04,300 WARN  org.dspace.identifier.doi.DataCiteConnector @ DataCite was unable to understand the XML we send.
2021-04-10 14:56:04,300 WARN  org.dspace.identifier.doi.DataCiteConnector @ DataCite Metadata API returned a http status code 400: param 'doi' required
2021-04-10 14:56:04,301 INFO  org.dspace.identifier.doi.DataCiteConnector @ We send the following XML:
<resource xmlns="http://datacite.org/schema/kernel-3" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dspace="http://www.dspace.org/xmlns/dspace/dim" xsi:
schemaLocation="http://datacite.org/schema/kernel-3 http://schema.datacite.org/meta/kernel-3/metadata.xsd"><identifier identifierType="DOI">10.34960/25</identifier><creators><creator><creatorName>Pim Reis</creatorName></creator></creators><titles><title>Sample</title></titles><publisher>My University</publisher><publicationYear>2021</publicationYear><contributors><contributor contributorType="DataManager"><contributorName>My University</contributorName></contributor><contributor contributorType="HostingInstitution"><contributorName>My University</contributorName></contributor></contributors><dates><date dateType="Accepted">2021-04-08</date><date dateType="Available">2021-04-08</date><date dateType="Issued">2021</date></dates><alternateIdentifiers><alternateIdentifier alternateIdentifierType="uri">http://hdl.handle.net/123456789/100</alternateIdentifier></alternateIdentifiers></resource>

Thank you!!