Currently it is not possible to remove the term “This is computer generated document and requires no signature”, as it is necessary to mention in the Invoice.
It seems like you have not added any signature in the application. If you are adding the signature to the Invoice, it shows as “This is digitally signed document”.
Please check the below link, to know how to add signature.
How to add new signature on Web?
After adding the signature, select the added signature as Default, so that the signature will be shown in all the future documents.
For previously created documents, you have to manually select the signature by editing the signature.