It is impossible to add metadata on some nodes, although they are instanceof ModifiableMetadataAwareAmetysObject.

The error returned is :
javax.jcr.nodetype.ConstraintViolationException: no matching property definition found for

testName

This situation happens for example on the nodes ametys:sites, ametys-internal:contents, ametys-internal:sitemaps, ametys-internal:resources, ametys-internal:zones...