Links to entries Library of Congress (LoC) are accepted when parsing a link from VIAF, but cannot be inserted directly. I have attempted to add this link: https://lccn.loc.gov/2020431442. MusicBrainz even shows a special icon for LoC, but it is impossible to choose “other databases” from the drop-down box.
Normally I insert links to LoC by parsing a relevant VIAF link, but in this case the reference is missing in VIAF, or at least I was not able to find it.
For comparison, I am perfectly able to directly insert a link to Katalog der Deutschen Nationalbibiothek (DNB, Katalog der Deutschen Nationalbibliothek). This behaviour is inconsistent: it should be possible to add a link to LoC directly the same way as it is possible to add a link to DNB.