Hi Aliaksandr,
Welcome to the group.
Re 1. at this point you cannot edit concepts because – as indicated by the green italics in your screenshot - you have chosen the “Validation” option at project creation. With validation enabled, your new input needs to be validated first by an authorized user. For more information see https://vocbench.uniroma2.it/doc/user/history_and_validation.jsf.
Re 2. To use DC Terms metadata, you need to owl:import the dct ontology first. This can be done in the Metadata > Namespaces and Imports section. For more information see https://vocbench.uniroma2.it/doc/user/namespaces_imports.jsf#imports.
Dublin Core Metadata can also be enabled at project creation.
Re 3. Note that „Some settings, in order to be edited, strictly require the project to be inactive, so the [„Edit projects editor“] is available only for closed projects.“ (https://vocbench.uniroma2.it/doc/user/projects.jsf#project_settings)
Hope this helps
Kind regards
Roland