In odoo V15,  remove 

<a t-if="has_button_access"
   t-att-href="button_access['url']"
   style="padding: 8px 12px; font-size: 12px; color: #FFFFFF; text-decoration: none !important; font-weight: 400; background-color: #875A7B; border: 0px solid #875A7B; border-radius:3px">
    <t t-esc="button_access['title']"/>
a>

from mail.message_notification_email

It's in the template "mail.mail_notification_paynow" which can be modified from the Views menu or writing a module.

In developer Mode go to settings->user interface->views, search for "Mail: Pay Now mail notification template"

Find the location in your view as shown below and add BOLD code inside as shown below which is just over the tag. 

By doing this way you just be sure that it will be hidden for Quotations only even if in different languages but code will be working for other mails like payments , etc. So you will not broke any running conditions. 

Best regards, 

Melih 

Eska Business Solutions

Thanks for the fast response.

When i go to "Technical -> User Interface -> Views" Unfortunately there is no mail.mail_notification_paynow