Looking at that error it looks like there is a Java version (the plugin is expecting Java 9 or there is a dependency that is) and/or dependency discrepancy with your plugin.
Please verify the right version of Java is being used to compile/build your plugin and there is no issue with any dependency that your plugin is using with regard to the Java version.
Kind regards,
Christopher Kochovski
Bitbucket Data Center Developer
The Java 8 version looks fine. Please do not use Java 9 as it is unsupported; see the Bitbucket supported platforms page for more details here
Are you aware of any dependency that your plugin is using that is/was built with Java 9? Or is there a specific version that you are importing that requires Java 9 potentially? Other community members had the same issue regarding dependencies, see here.
Kind regards,
Christopher Kochovski
Bitbucket Developer