This error could have occurred when you manually try to alter the external cache database using the VQL command. I would use the graphical tool rather than VQL command to configure the cache database.
In addition to that ensure that you have defined the datatypes that are supported by the Exasol database in the ‘cacheConfig-generic.xml’ file.
The option ‘can be data movement target’ would be enabled for the databases that are officially supported by the cache engine.
You could refer the [Generic Support for Other Databases](https://community.denodo.com/docs/html/browse/6.0/vdp/administration/cache_module/generic_support_for_other_databases/generic_support_for_other_databases#generic-support-for-other-databases) and [Cache Module](https://community.denodo.com/docs/html/browse/6.0/vdp/administration/cache_module/cache_module#cache-module) section of the Virtual DataPort Administration Guide for more details on configuring the generic database and the cache databases that supports Data Movement respectively.
Hope this helps you!