PdfMerger is deprecated and was removed in pypdf (version 5.0.0). So if you are using latest version of pypdf, you will need to use the PdfWriter class from the pypdf module.

from pypdf import PdfWriter

merger = PdfWriter()

for pdf in ["file1.pdf", "file2.pdf", "file3.pdf"]:
    merger.append(pdf)

merger.write("merged-pdf.pdf")
merger.close()

You may refer to this link: https://pypdf.readthedocs.io/en/stable/user/merging-pdfs.html