'One or more packages have not been loaded (For complete information see Log)' after importing the Tablespace Assignment package (4236471)

The package is dependent on common packages for MySQL, PostgreSQL and Oracle.

In TDM 5.3, all of these packages were automatically enabled (if installed).
In TDM 5.5, all database packages are installed, but they can be enabled/disabled.

Please check the Settings Menu | Add/Remove Databases and enable at least one version of each database platform (MySQL, PostgreSQL, Oracle). This will enable the common packages as well and it should then be possible to load the custom package.

The TablespaceAssignment package has not been loaded most likely because it lacked the packages it is dependent on, that is Oracle/PostgreSQL/MySQL packages (which are enabled/disabled when the respective databases are enabled/disabled). If one of these was not enabled, it can cause the error.