I have a custom button which instigates Apex to create a PDF and attach it to the record - this works. What I now want it to attach the PDF to a related object (child/parent relationship) but struggling with the ParentID - I underlined where the error is occurring.
Here is the code I have 
public with sharing class SaveAndOpenPDF {   public String recordId {     get {       return ApexPages.currentPage().getParameters().get('Id');     }   }   // this is to make testing a lot easier -- simply append renderAs=html   // to the url parameters to test, add displayOnly=1 if you don't want to save   public String renderAs {     get {       if (String.isBlank(ApexPages.currentPage().getParameters().get('renderAs'))) {         return 'pdf';       } else {         return ApexPages.currentPage().getParameters().get('renderAs');       }     }   }   public PageReference saveAndOpenPDF() {     if (String.isBlank(ApexPages.currentPage().getParameters().get('displayOnly'))) {       Id attachmentId = savePDF();       return openPDF(attachmentId);     } else {       return null;     }   }   public Id savePDF() {     Attachment attachment = new Attachment();     attachment.ParentId = 'aFA.Id';     attachment.name = 'PDF_'+String.valueof(Datetime.now())+'.pdf';     PageReference pdf = Page.SaveAndOpenPDF;     pdf.getParameters().put('Id', recordId);     pdf.getParameters().put('displayOnly', '1');     pdf.setRedirect(true);     try {       attachment.Body = pdf.getContent();     }     catch (VisualForceException e) {       attachment.Body = Blob.valueof('There was an error.');     }     attachment.ContentType = 'application/pdf';     insert attachment;     return attachment.Id;   }   public PageReference openPDF(Id attachmentId) {     PageReference ret = new PageReference('/servlet/servlet.FileDownload?file=' + attachmentId);     ret.setRedirect(true);     return ret;   } }