Hello @asma,

Go to your report's paper format and set header_line as False.

if you have not created custom paper format create custom one and set it in your report
like this after that you can set attribute as your need. 

<record id="paperformat_custom"  model="report.paperformat">
 <field name="name">A4 Custom</field>
 <field name="default" eval="True"/>
 <field name="format">A4</field>
 <field name="page_height">0</field>
 <field name="page_width">0</field>
 <field name="orientation">Portrait</field>
 <field name="margin_top">5</field>
 <field name="margin_bottom">13</field>
 <field name="margin_left">7</field>
 <field name="margin_right">7</field>
 <field name="header_line" eval="False"/>
 <field name="header_spacing">15</field>
 <field name="dpi">90</field>
</record>