If you are using SAP Cloud Integration on Cloud Foundry then you probably have noticed you cannot add the default ESBMessage Send (MessagingSend) role to your user.
You will need to create a role collection to support this.
In your BTP cockpit go to the Role Collection
Select Add (Plus button)
Give it a name
Open the newly created role
Select Edit
In the Role Name search for MessagingSend
Select it and press Add
Then you can search for the user you want to assign the role

Notice: There can be some delay in the roles assignment once you add the role to new users or remove it.