Please know that to add attributes in opencart we have to follow below instructions.
The attributes section lets you edit the attributes compared between products in "Product Comparison". When viewing products in the store front, customers have the option to add products to "Add to Compare" to analyze the similarities and differences between products in the store. Attributes can be used to describe the size, weight, color, and any other specifications of the product that have potential for comparison. You can create these attributes in the Catalog > Attributes area. To add attributes directly to products, visit the Attribute tab under Products.
An attributes group contains related attributes that can be compared under Product Comparison. Attributes such as 'Clockspeed' and 'No. of Cores' can be organized under the larger attribute group of 'Processor'.
To create an attribute group, go to Catalog> Attributes> Attributes Group and click "Insert". This form only requires an attribute group name in "Attribute Group Name". Adding the "Sort Order" is optional. Adding a sort order will affect the arrangement order of attributes under Product Comparison. The attribute groups created here can have individual attributes added to them later.
In Catalog > Attributes, attributes can be created, edited, and deleted.
To create a new attribute, click "Insert". This button will direct you to a page asking for the "Attribute Name", "Attribute Group", and "Sort Order".
The "Sort Order" determines the position of the attribute if the product has multiple attributes assigned to
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