I am the x6 owner's license like you. I made it for you first, but I recommend you to look at the tools below. I've been using this.
Certificates and signatures are things that you are afraid of installing unsafe file settings. In order to avoid a series of troublesome things such as certificate signing, we can use the direct signing tool of mobile phone.
You can download it from the computer, put it in the designated location, and then you can install s60sign (Saipan Certificate Free Direct Sign Tool) on your mobile phone (check it online, then download your x6 version, find the installation program of the previous tool through file management, and click Install). After the installation, run the software, find the program you want to install through the software, click, and then you can sign the program. Then he will prompt to install, and it will be ok! This applies to all software that cannot be installed directly without a certificate or with an expired certificate or an error.
In addition, I tell you that SIXX is generally a program that can be installed directly, and jar programs do not need to be signed. Sis or SIXX shows that the certificate is wrong and needs to be signed.