In chapter 19.9. USER OPERATIONS, there are different commands used in example then it's written.
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Adding client roles to a user
There is a dedicated add-realm operation that can be used for adding both realm roles, and client roles.

$ kcadm.sh add-roles -r demorealm --uusername testuser --cclientid realm-management --rolename create-client --rolename view-users

---command in description should probably be "add-roles"
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Removing client roles from a user
There is a dedicated remove-realm operation that can be used for removing both realm roles, and client roles.

$ kcadm.sh remove-roles -r demorealm --uusername testuser --cclientid realm-management --rolename create-client --rolename view-users

---command in description should probably be "remove-roles"