your check box id should be unique
<asp:DataList ID="dtEmpCategory" runat="server" RepeatDirection="Horizontal" RepeatColumns ="8">
<ItemTemplate><div id="boxcolcb" >
<p>
<asp:CheckBox ID="ChkEmployee" runat="server" CssClass="abc"
Text= '<% #Eval("EmployeeCategoryName") %>' oncheck="fun(this.id)" />
</p>
<p>
<asp:TextBox ID="txtEmployeeIncentive" runat="server" MaxLength="7"
ondrop="return false" placeholder="Up to 7 digits"
attr-identity="txtEmployeeIncentive" onpaste="return false" oncut="return false"
style="display:none;"></asp:TextBox>
</p>
</div> </ItemTemplate>
</asp:DataList>
<script type="text/javascript">
function fun(id)
{
id="<%="+id+".ClientID%>"
$(id).attr("display", "block");
}
</script>