Hi Elton,
this is an internal error in BRFplus that might be caused either by the XML import or an erro in the activation logic in BRFplus.
What you can do is use the check functionality in the BRFplus workbench to find any invalidities in the applications that have been imported that might help to find the cause for that.
Anyway, there is not really much you can do except for opening an OSS message to SAP support and let them check the situation
BR
Christian
P.S: According to the blog Output Management via BRF+ it seems to work, so I am quite sure that the cause is a missing note in your system