The method assign() does not exist on Magento\Framework\View\Element\BlockInterface. It seems like you code against a sub-type of said class. However, the method does not exist in Magento\Framework\View\T...DataSourcePoolTestBlock or Magento\Framework\View\E...ontrol\ControlInterface or Magento\Framework\View\E...nent\ContainerInterface or Magento\Framework\View\T...Unit\BlockPoolTestBlock or Magento\Ui\Block\Compone...epsWizard\StepInterface or Magento\Framework\View\E...nt\UiComponentInterface or Magento\Framework\View\E...onent\JsConfigInterface or Magento\Framework\View\E...ent\DataSourceInterface or Magento\Ui\Component\Form\Element\ElementInterface or Magento\Ui\Component\For...aType\DataTypeInterface or Magento\Framework\View\E...t\BlockWrapperInterface or Magento\Ui\Component\Lis...Columns\ColumnInterface. Are you sure you never get one of those?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
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->/** @scrutinizer ignore-call */ assign('header', __('Add New Sub Seller'))
This function has been deprecated. The supplier of the function has supplied an explanatory message.
The explanatory message should give you some clue as to whether and when the function will be removed and what other function to use instead.