Disregard the issue below — Global Settings icon does appear in post & page content editing, but does not appear in Site Editor screen, perhaps by design.
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Using v3.7.0 and v3.7.1 Stackable Global Settings icon and settings are not available on the WordPress Core v6.1 Site Editor screen (using Twenty Twenty-Two FSE theme, Gutenberg disabled–though previously enabled). Nor is Stackable listed in Site Editor’s Plugin section.
Note no js error in console, nor any other noted conflict. This is also after disabling all plugins other than Stackable and using Twenty Twenty-Two theme.
In addition, the Twenty Twenty-Two’s theme colors are not selectable in many blocks, core (non-Stackable) and Stackable, which is potentially due to the Stackable Global Colors and Typography feature — perhaps the “Use only Stackable colors” feature is enabled, though without access to Stackable Global Settings, that can not be checked or manipulated.
Stackable Global Colors reference: https://wpstackable.com/blog/global-colors-and-global-typography/