Unfortunately this is not possible. After a signing request has been sent, the added signers can no longer be removed. However, a transaction can be edited and you can change a signer. By entering a different e-mail address, you can, for example, have an internal person sign and still complete the transaction. Another option is to create a completely new transaction. Below you can see how you can edit the transaction.
You go to the relevant transaction and click on "Edit" (blue button at the top right).
You must then additionally confirm that you want to edit the transaction. As long as you do not change anything in the document, any signatures already placed will remain valid. If you do make changes to the document, all signatories will receive a new request to sign. Only the modification of a signer does not affect the validity of signatures that have already been entered.
After you have specified another signer, click "Next" at the top right. You will then return to the 'Designer'. You may still be able to make changes here, but please note that if you do this, all signatories must sign the document again. If you click on "Send to Sign" without modifying the document, the new signer you just added will receive a signing request, and the transaction can be completed once everyone has signed.