Confusingly enough, there is (1) no error code, just a "Failed to share, the stylex file is empty or does not contain valid symbols" in red at the bottom and (2) no option to Analyze in the Share As Web Style tool. There also isn't a way to select what type of features are in the style (2d, 3d, dictionary). The style was created by just saving existing symbols from the symbology tab.
There are only 2D Point symbols in the style. I went through them in the Catalog/Properties tab to verify and look for any weird inconsistencies. I couldn't find any.
In the Error link you provided, the option for 2D says to click New Item to add a supported 2D style. That isn't an available option. Add and Import are both greyed out while New is possible. Do you have any ideas on why the existing symbols could be considered invalid, or why Add item isn't an available option?
Thanks!