Apex variables are not part of Javascript runtime, they are rendered from server side.
- You need to enclose the Apex variables with single quote in client side.
Try this:
window.open('/apex/FormTemplatePrinterRequest?'+'id='+'{!Form_Requests_abv__c.Id}'+'&TemplateId='+'{!Form_Requests_abv__c.Form_Template_Name_abvId__c}','_blank');}
Even though above code will work its better to JSENCODE the Apex variables to avoid XSS vulnerablity:
window.open('/apex/FormTemplatePrinterRequest?id={!JSENCODE(Form_Requests_abv__c.Id)}&TemplateId={!JSENCODE(Form_Requests_abv__c.Form_Template_Name_abvId__c)}','_blank');}
If you apply for security review, all these vulnerabilities will raise serious red flags, so its better to follow it.