Although you don't have a degree certificate but have a diploma of the second degree, you have completed the study of the second degree and obtained the diploma, which shows that you have the relevant knowledge and skills. So this learning experience can still be written into your resume.

In your resume, you can describe this learning experience in detail, including your second degree major and the courses you took, as well as the skills and experience you gained during your study. This information can help employers understand your professional background and learning ability, so as to better evaluate your job qualifications.