Something like:

"INSERT INTO tblVoids
(VoidId, VoidCheckID,
VoidItemID,VoidAmount,VoidReasonID,VoidManagerID,VoidQTY)
SELECT tblCheckDetails.VoidId, tblCheckDetails.SalesId,
tblCheckDetails.ItemId, tblCheckDetails.Amount,
tblCheckDetails.ReasonId, tblCheckDetails.ManId, 1 FROM tblCheckDetails
WHERE tblCheckDetails.SalesId = " & Forms!frmFXVoidItem!txtSalesId

For every record in tblCheckDetails that meets the SalesID criteria, the
following would add one record to tblVoids.

This is what you asked for (I think...), but I have no idea if this is
really what you want to do since I have no idea what the relationships
between those tables are or how they relate to the 2 forms you are pulling
data from.

HTH,

Dim db as DAO.Recordset
Dim rs as DAO.Database

Set db = CurrentDB
Set rs = db.OpenRecordset("SELECT [SalesID] FROM tblCheckDetails WHERE
[SalesID] = " & Forms!frmFXVoidItem!TxtSalesID)

Do While Not rs.EOF
   CurrentDb.Execute "INSERT......<your code>

   rs.MoveNext
Loop

Dear DS.

Probably, you want something like this:
CurrentDb.Execute "INSERT INTO tblVoids (VoidID,VoidCheckID," & _
"VoidItemID,VoidAmount,VoidReasonID,VoidManagerID,VoidQTY)" & _
" SELECT VoidID,SalesID,ItemID,Amount,ReasonID,ManID,1 FROM" & _
" tblCheckDetails WHERE SalesID=" & Forms!frmFXVoidItem!TxtSalesID