Sorry, it's a different one. Here is solution for similar problem provided in other ticket:
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In a nutshell, you can't register a new `mobile` attribute because its metadata is indeed already present, though it's not shown in web UI.
You need to find metadata of that attribute in LDAP (it's under `ou=attributes` branch) and change value of its attribute (i.e., attribute of the metadata of the attribute, that's it) named **"gluuAttributeOrigin"** to **gluuCustomPerson**.
Here are steps that should help (all are executed inside container):
1. Put your LDAP pass (it's same as your default admin pass) into `/tmp/.pw`
2. Find out DN of `mobile` attribute's metadata entry: `# /opt/opendj/bin/ldapsearch -h 127.0.0.1 -p 1636 -s sub -T -Z -X -D 'cn=directory manager,o=gluu' -j /tmp/.pw -b 'o=gluu' '&(gluuAttributeName=mobile)(objectClass=gluuAttribute)' dn`
3. Create `~/change_registered_attr.ldif` file with next content:
```
dn: here,goes,mobile,dn
changetype: modify
replace: gluuAttributeOrigin
gluuAttributeOrigin: gluuCustomPerson
```
Now apply this ldif: `# /opt/opendj/bin/ldapmodify -h 127.0.0.1 -p 1636 -Z -X -D 'cn=directory manager,o=gluu' -j /tmp/.pw -f ~/change_registered_attr.ldif`
You now should see `mobile` on "Attributes" page, and it should appear on **"gluuCustomPerson"** tab when you create SAML TRs. You still may need to make it active, though (done in web UI)
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So, in your case you need to substitute `mobile` for `cn`.