Is there a value already in the mkspecs box? I think the instruction means, "leave it at the default value", not "change the value to default." Where this value appears depends on exactly which Qt Creator version you have
The Welcome plugin is missing because the command you launch Qt Creator with tells it not to load that plugin. This is because the Pi does not have the necessary OpenGL or other graphics support the plugin requires. Loading it will probably leave a black screen in Creator or hang it totally.
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