Understanding where you are in your code is crucial, and having clean code makes this process much easier. It seems like there was an extra div element that shouldn't have been there:
<div class="container-fluid">
<div class="row">
<div class="col-sm-2 col-md-2 col-lg-2" style="border-bottom:double;">
<ul class="nav flex-column" style="margin-left: -15px;">
(navigation menu)
</ul>
</div>
<div class="col-sm-10 col-md-10 col-lg-10" style="border-bottom:double">
<div class="row">
<div class="col-3">
... an invisible panel...
</div>
<div class="col-12">
... panel with search option for populating repeater ...
</div>
<div class="col-12">
<asp:Panel ID="panel_endiaferomenwn_provoli" runat="server">
<div class="table-responsive table-bordered">
<table style="min-width:950px; width:100%">
<tr>
... table contents ...
</table>
</div>
<asp:Repeater ID="rep_all" runat="server" DataSourceID="data_rep" OnPreRender="rep_all_PreRender">
<ItemTemplate>
<div class="table-responsive table-bordered">
<table style="min-width:950px; width:100%;">
<tbody>
... details and data binding ...
</tbody>
</table>
</div>
</ItemTemplate>
</asp:Repeater>
<asp:SqlDataSource ID="data_rep" runat="server" (connection details)></asp:SqlDataSource>
</asp:Panel>
</div>
</div>
</div>
</div>