We'll be using planting work as an example.
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Once you are logged in to your account navigate to the planting work list as shown below.

If you try to delete the existing planting work with subforms from the list by clicking on the delete symbol you will get an error message with a failure to delete. 

From the list view in the "Update" section, click on the green edit symbol of the planting work you are trying to delete.

Then delete all the subforms in that planting work. 

Once all the subforms are deleted, click "Save" at the bottom of the page. 

Back in the list view click on the red delete symbol.

Now the record has been deleted.