Try this sample. But I am notsure why do you want to make the cell editable.

ListBox and GridView

<asp:ListBox ID="ListBox1" runat="server">
            <asp:ListItem>Manager1</asp:ListItem>
            <asp:ListItem>Manager2</asp:ListItem>
            <asp:ListItem>Manager3</asp:ListItem>
            <asp:ListItem>Manager4</asp:ListItem>
        </asp:ListBox>
        <asp:GridView ID="UserAllocationGrid" runat="server"
            AutoGenerateColumns="False">
        <Columns>

            <asp:BoundField DataField="Manager" HeaderText="Manager"
                SortExpression="managers" />
                <asp:TemplateField HeaderText="Allocation Percentage">
                <ItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server"
                        Text= '<%# Bind("AllocationPercentage") %>' BorderStyle="None"></asp:TextBox>
                </ItemTemplate>
                </asp:TemplateField>

        </Columns>
        </asp:GridView>

Code Behind

 void fillGV()
    {
        DataTable UserAllocationTable = new DataTable();
                 
        UserAllocationTable.Columns.Add("Manager");
        UserAllocationTable.Columns.Add("AllocationPercentage");
        // go through listbox1 to find selected managers = selectedManagersList 
        List<string> selectedManagersListDates = new List<string>();
        int counterR = 0;
        foreach (ListItem strItem in ListBox1.Items)
        {                     
                //selectedManagersListDates.Add(strItem.Value); 
                DataRow drManagerName = UserAllocationTable.NewRow();
                UserAllocationTable.Rows.Add(drManagerName);
                UserAllocationTable.Rows[counterR]["Manager"] = strItem.Value;
                counterR = counterR + 1;            
        }
       // ViewState["UserAllocationTable"] = UserAllocationTable;
        UserAllocationGrid.DataSource = UserAllocationTable;
        UserAllocationGrid.DataBind(); 
    }

Use this void in any event I did it in abutton click

  protected void Button1_Click(object sender, EventArgs e)
    {
        fillGV();
       
    }

Please note the changes I made to your code