You can make certain edits to your Sales Orders depending on the Stage of the Sales Order. Please visit the following article on what you can edit based on the Sales Order stage:
Edit a Sales Order
As mentioned in the above article, within the Draft and/or Active stage of the Sales Order, you can edit all parts of your sales order, such as adding or removing line items.
Once you are at the Finalized stage of the Sales Order, you CANNOT remove items from the order. However, you can remove the line items from the Finalized Stage of the order if the following conditions are met:
1. The items you are looking to remove from the Sales Order have not yet been Invoiced.
2. The items you are looking to remove from the Sales Order have not yet been Fulfilled.
If you have deleted the Fulfillment from a Fulfilled Sales Order to delete line items from the order, this cannot be done as there has been a stock movement already (fulfillment). The only work-around here is to Void the original Order, Clone the Sales Order, Remove the unwanted items from the order, finalize/Invoice, and fulfill the Sales Order as usual.
Once the line items on the Sales Order have been Fulfilled, you can no longer remove them from the order. There aren't any workarounds for this. To rectify, please Create a Return on the Sales Order.