Dear penny23232
To fix this issue, please go to modules/mod_sj_k2_extra_slider/mod_sj_k2_extra_slider.xml and find:
<field name="item" type="item" default="" label="K2_ADD_ITEMS" description="K2_USE_THIS_TO_ADD_ITEMS_TO_THE_LIST_BELOW"/>
<field name="items" type="items" default="" label="K2_DRAG_AND_DROP_TO_REORDER_ITEMS" description="K2_DRAG_AND_DROP_TO_REORDER_ITEMS_CLICK_THE_REMOVE_ICON_TO_REMOVE_AN_ITEM_FROM_THE_LIST"/>
=> change to:
<field name="items" type="k2modalselector" default="" label="K2_ADD_ITEMS" description=""/>
thanks