Please check your license expiry date which should not be less than the release date of version 17.8. However, if the license is expired, then Aspose.Pdf for .NET API shows an error for renewal of the license. You can open license file with any editing tool like notepad and see subscription expiry tag value. In order to test the license file, please create a small project application with the following code, and then let us know how that goes into your environment.
// apply license
com.aspose.pdf.License pdfLic = new com.aspose.pdf.License();
// initialize a new PDF
Document document = new Document();
// add a new page
// save pdf