You are calling a subform. You need to also reference the parent form.

These videos should help with the syntax:
Value From a Form
Value From Subform

Here's the Code:

Private Sub BtnSaveRecordUsage_Click()
    
    CopyDataToContinuousForm

End Sub

Sub CopyDataToContinuousForm()

    Dim frmSingle As Form
    Dim frmContinuous As Form
    
    ' Set references to the SingleForm and ContinuousForm
    Set frmSingle = Forms("SubtractRecordUsageF")
    Set frmContinuous = Forms("TractedInventoryUsageSubF")
    
    ' Copy data from SingleForm to ContinuousForm
    With frmContinuous.RecordsetClone
     .AddNew
    .Fields("ProductID").Value = frmSingle.Controls("ProductID").Value
    .Fields("TransactionDate").Value = frmSingle.Controls("TransactionDate").Value
    .Fields("UsageAmount").Value = frmSingle.Controls("TransactionQty").Value
    .Fields("TransactionType").Value = frmSingle.Controls("TransactionType").Value
    
    ' Copy other fields as needed
    .Update
    End With
    
    ' Go to a new record in ContinuousForm
    DoCmd.GoToRecord acDataForm, "TrackedInventoryUsageSubF", acNewRec
    
    ' Insert the data into the new record
    With frmContinuous.RecordsetClone
    .Fields("ProductID").Value = frmSingle.Controls("ProductID").Value
    .Fields("TransactionDate").Value = frmSingle.Controls("TransactionDate").Value
    .Fields("UsageAmount").Value = frmSingle.Controls("TransactionQty").Value
    .Fields("TransactionType").Value = frmSingle.Controls("TransactionType").Valuee
    
    ' Insert other fields as needed
    .Update
    End With

End Sub

I am trying to write VBA Code in Microsoft Access to past Data from Fields on an Single Form to an Continuous Form, than go to new record and insert the Data into the Continuous Form on another Single Form.