VC2090: core- names are reserved for system blocks

Names starting with core- belong to system blocks: the built-in blocks every site has, like core-button and core-site-header. A site block with a core- name is only allowed as a customization of one of them, and there's no system block by this name.

How to fix

Rename the block, for example site-button instead of core-buton. To customize a system block, use its exact name; Insert Block lists them under System blocks.