SWitchport mapper add switch

SWitchport mapper add switch

Hi experts !
recently i`ve started to work with an installed opmanager system, its licensed with 150 devices and never expire time limit.
i wanted to add switch to system but add button is completely disabled i suspected that if there are some limitation on number of devices (previousely there are about 30 devices added) and i delete some devices but new device still can not be added (add button is disabled) can you tell me why it happening ?
thanks a lot
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