Just looking for some more details.  Assuming you’re trying to do this in Nintex Forms.

Which version of SharePoint and Nintex Forms are you using?

Are you using the Classic or Responsive forms designer?

Are there any actions or conditions that cause the text field to be cleared?

Which version of SharePoint and Nintex Forms are you using?-SharePoint 2019

Are you using the Classic or Responsive forms designer?- Responsive

Are there any actions or conditions that cause the text field to be cleared?-

When the form is opened I have a default string in a single text mandatory control field that is hidden in a panel control field. The panel is hidden or opened by a Yes\No control which default value is No and hidden. What I want to happen is when Yes is selected by the Panel Control the default string should be removed when the panel opens

You can create a rule on the Single Line Textbox control to clear out any text entered into that field. Follow these steps in the Responsive designer:

1. Select the Single Line Textbox control.

2. Click on the “Add Rule” button in the top Ribbon.

3.. Configure the new rule.

• Rule Name = Clear Information
• When = Open Panel (Open Panel is the name of the Yes/No control. Click the Formula Button to the right and go to the Names Controls tab to select the control.)
• Then = Set Value
• Value = “”  (double quotation marks = empty)

This will empty any value entered or defaulted in the Single Line Textbox control inside the Panel.