We have to check the first the item fields which are locked. So we can check by
Customization > List, Record & Fields > Item Fields
If the item field is locked then we have to check whether it is using in the Fields Sets or not. If it is using in the fIeld sets then we foirst have to remove the item fields from there.
Also remove from the Search Index if it’s there
Commerce > Website > Website List > Subtab(Field Sets)
Also remove from
Commerce > Website > Website List > Tab(Search Index) > Subtab(In any of the fields for eg: Facet Fields)