ASP.net DropDownList, SelectedIndexChanged Not Working - asp.net

Question

I tried every way but it didn't work at all. I would be glad if you can help. Trigger, AutoPostBack etc. I added everything, but still could not reach the solution in any way. Thank you.
<%# Page Language="C#" AutoEventWireup="true" CodeFile="prequest.aspx.cs" Inherits="musteri_prequest" validateRequest="false" enableEventValidation="false" viewStateEncryptionMode="Never" %>
<form id="form-login" class="text-start">
<form runat="server">
<asp:ScriptManager runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<contenttemplate>
<div class="mb-3">
<label class="form-label" id="lbl-nombre" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Cihaz Seçiniz:</label>
<asp:DropDownList ID="prequ_cihaz" runat="server" AutoPostBack="True" EnableViewState="true" CssClass="form-control" style="border-color: var(--first-color);margin-bottom: 15px;" OnSelectedIndexChange = "Prequ_cihaz_SelectedIndexChanged"></asp:DropDownList>
<label class="form-label" id="lbl-nombre-1" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Problem Türü:</label>
<asp:DropDownList ID="prequ_prob" runat="server" CssClass="form-control" style="border-color: var(--first-color);margin-bottom: 15px;"></asp:DropDownList>
</div>
<div class="mb-3"></div>
<div class="mb-3"></div>
<div class="mb-3">
<asp:Label ID="prequest_label" runat="server" Text="" style="color:rgb(148,0,211); font-family: Alatsi, sans-serif; margin-bottom: 9px;" ></asp:Label>
<label class="form-label" id="lbl-nombre-1" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Probleminizi açıklayınız.</label>
<asp:TextBox ID="prequest_exp" runat="server" TextMode="MultiLine" Rows="5" CssClass="form-control form-control-lg" style="border-color: var(--first-color);margin-bottom: 15px;"></asp:TextBox>
</div>
<div class="text-center">
<asp:Button ID="prequest_button" runat="server" onClick="prequest_button_Click" Text="Gönder" CssClass="btn btn-primary text-center" style=" background: var(--bs-indigo);" />
</div>
</contenttemplate>
</asp:UpdatePanel>
</form>
protected void Prequ_cihaz_SelectedIndexChanged(object sender, EventArgs e)
{
a = prequ_cihaz.SelectedIndex;
prequest_exp.Text = "Run!";
}

Answer 1

I found out that the error was due to two nested <form> tags.
Solution:
<form runat="server" class="text-start" style="max-width: 70%; margin: 0 auto; margin-top: 20px;">
<div class="mb-3"><label class="form-label" id="lbl-nombre" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Cihaz Seçiniz:</label><asp:DropDownList ID="prequ_cihaz" runat="server" AutoPostBack="True" EnableViewState="true" CssClass="form-control" style="border-color: var(--first-color);margin-bottom: 15px;" OnSelectedIndexChanged = "Prequ_cihaz_SelectedIndexChanged">
<asp:ListItem>Cihaz Seçiniz</asp:ListItem></asp:DropDownList>
<label class="form-label" id="lbl-nombre-1" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Problem Türü:</label><asp:DropDownList ID="prequ_prob" runat="server" CssClass="form-control" style="border-color: var(--first-color);margin-bottom: 15px;"></asp:DropDownList></div>
<div class="mb-3"></div>
<div class="mb-3"></div>
<div class="mb-3"><asp:Label ID="prequest_label" runat="server" Text="" style="color:rgb(148,0,211); font-family: Alatsi, sans-serif; margin-bottom: 9px;" ></asp:Label><label class="form-label" id="lbl-nombre-1" for="txt-nombre" style="font-family: Montserrat, sans-serif;font-size: 15px;font-weight: bold;">Probleminizi açıklayınız.</label><asp:TextBox ID="prequest_exp" runat="server" TextMode="MultiLine" Rows="5" CssClass="form-control form-control-lg" style="border-color: var(--first-color);margin-bottom: 15px;"></asp:TextBox></div>
<div class="text-center"><asp:Button ID="prequest_button" runat="server" onClick="prequest_button_Click" Text="Gönder" CssClass="btn btn-primary text-center" style=" background: var(--bs-indigo);" /></div>
</form>