You get the following error message during a Shopware update or when deactivating or uninstalling a theme:
Unable to deactivate or uninstall theme "{themeName}". Please remove the following theme assignments: {assignments}.
As the error message says, a theme must no longer be assigned to a sales channel so that it can be deactivated or uninstalled.
With this message, Shopware asks you to remove the assignment of a theme (see {themeName}) from the sales channel (see {assignments}).
In this case, simply assign the Shopware Standard Theme to the corresponding sales channel.