Why can't I find a CERT.RSA file in my Android APK?
Sometimes the "CERT.RSA" file is called something else. No idea why. I have often found the certificate file to be called something".RSA".

For example, when I decompile the "wp.wattpad" APK, it has its certificate saved as "BNDLTOOL.RSA"

My advice to you would be to find the file in the META-INF folder that has the extension ".RSA". It is probably the certificate.