The trait ByTIC\DocumentGenerator\...fLetters\PdfLetterTrait requires the property $violations which is not provided by ByTIC\DocumentGenerator\...ls\PdfLetters\PdfLetter.
The expression return $this->issueDate could return the type null which is incompatible with the type-hinted return DateTime. Consider adding an additional type-check to rule them out.
The property issueDate does not exist on ByTIC\DocumentGenerator\...ls\PdfLetters\PdfLetter. Since you implemented __get, consider adding a @property annotation.