UAGB refers to Ultimate Addons for Gutenberg, a plugin that creates custom blocks for Gutenberg, the block-editor.
If you wish to use the uagb/section block (which uses both Gutenberg and UAGB), you’ll need to install and activate the Ultimate Addons for Gutenberg plugin.
If you already have that plugin installed and activated and the error still appears, the error is specifically with that plugin and their staff would be able to best assist you 🙂