(Reported by S. Fanning) Add Tables dialog. Dialog accessed by selecting Tools > Relationships on the Menu bar. Help page reports “Could not find Help page (404)”. Reported Help ID: dbaccess/ui/tablesjoindialog/dialog-action_area1.
I confirm this report (with both LO 7.3.3.1 and with LO 7.4b1).
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/a8a10f51d66795e9fd8f49839d01e26c9cc4e2a2 tdf#149222 Missing Help page for database Relationship dialog