Hi,

If you modified the footer using the "Configure Your Document Layout" form and added a custom footer but now can’t delete or remove it, the issue likely comes from changes saved in the QWeb template. To fix this, go to Settings > Technical > User Interface > Views, search for the view named something like external_layout_standard, open it, switch to Developer Mode if not already, and manually remove or edit the footer code. For a detailed step-by-step guide to modify footer section, refer to this blog:

https://www.cybrosys.com/blog/how-to-customize-header-footer-for-all-reports-in-odoo-15

Hope it helps