Can you tell me, which graph you’ve extended, which event? Or it was customization plugin extension?

The actual content of the page doesn’t matter. You can create a new ‘Code’ page and paste only that line in and it will break.

I tried to find class ApplyDocuments in Acumatica source code, but failed. I’m on the build 22.206.0035 . The code, which I’ve written goes below:

namespace PX.Objects.SO
{
  public class SOOrderEntry_Extension : PXGraphExtension<PX.Objects.SO.SOOrderEntry>
  {
    #region Event Handlers

    public virtual void _(Events.RowUpdated<SOOrder> e)
        {
            var amountOfPrepayments = 123;
            ApplyDocuments.SetCaption($"Apply Prepayment of ${amountOfPrepayments}");
        }

    #endregion
  }
}

any idea what I’m missing?

ApplyDocuments is a custom action I defined on the ARInvoiceEntry graph, but I don't think that had any impact on the issue. I was on an earlier build, but I don't know which one at the moment. Maybe it was fixed in a later build?

Without seeing actual source code, I can only guess.