I was gaming Girls symphony, and suddenly this board appears :imgur.com/oNbLnoc
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The popup basically tells you that your equipment inventory is full.
In the Character Management, click on the double cycle arrow at the left and either spend some Sound Crystals to expand the inventory or use them to upgrade equipment.
If you wanted a faster response, you should post your questions in our Discord channel.
(Edited by Ar-cen-ciel)