Hi Syaza,
Welcome to MS Q&A
As per the similar thread https://learn.microsoft.com/en-us/answers/questions/514432/selectpdf-error-on-azure-app-service
As mentioned in SelectPdf docs:
SelectPdf Html To Pdf Converter (v18.3) or above works on Azure Web Apps, on Windows, and an App Service Plan starting with Basic (does not work with Free/Shared plans). Using a Standard or Premium plan will make conversions faster. The Web Apps version uses a restricted rendering engine and because of that, some features are not available. To name a few: no support for web fonts, support only for single page PdfHtmlElement objects, no support to exclude elements from conversion.
If the error persists, request you to open an issue with SelectPdf on GitHub here: https://github.com/selectpdf/selectpdf-free-html-to-pdf-converter/issues/
If an answer is helpful, please "Accept answer" and/or "Up-Vote" which might help other community members reading this thread.
Thanks
Deepanshu