I use DevExtreme from DevExpress to build multiplatform app.
Since DevExtreme doesn’t support anymore its building tool, I use it only to create a zip package and then I build it with build.phonegap.com.
I had to upload my p12 file in this site, but first I had to convert it correctly.
This command line reached the goal:
keytool -v -importkeystore -srckeystore android_certificate.p12 -srcstoretype PKCS12 -destkeystore myKey.keystore -deststoretype JKS
After you run this command, in the output you’ll see the certificate alias. Use this to import the file in build.phonegap.com. If this is wrong, you’ll get an error while compiling the apk.