Ubuntu cannot open shared library mysqlx.so
Three comments:

One: Maybe you are using  'mysqlx.so; and you forgot the closing " ' "

I mean please use 'mysqlx.so' in stead of 'mysqlx.so

Two: We expect to find the file mysqlx.so in the that /usr/lib/mysql/plugin/.

Is the file mysqlx.so in than path /usr/lib/mysql/plugin/

Three: If so, maybe you have to check if the file mysqlx.so has full permissions for the user that controls mysql.