Error: Insufficient Access to Update OpportunityLineItem fields: readymade_inv_Inventory_Item_Plus__c. 

The above error generally occurs when a user tries to add a new product to the opportunity by a user which does not have access to the Inventory Item field. You can fix it by adding the Modify All permission for:

• Object: Opportunity Product
• Field: Inventory Item (readymade_inv__Inventory_Item_Plus__c)

This can be done directly on the profile, or by creating a new permission set here.