Like version 12 just add debug=1after /web in url like below

http://localhost:8069/​web?debug=1#action<other url details>

Just an additional comment, the "General Settings" won't appear if you haven't enabled any apps. So if you just see "Users & Companies" in the "Settings" menu, enable ANY one app (e.g. CRM.) Then follow the instructions by Niyas (10/15/19) or Odoo Tools (10/15/19)

Hi,

You can see it here: How To Activate Developer Mode in Odoo13

Thanks

Go to the Settings > General Settings > Scroll to the very bottom > Find the section 'Developer Tools'

All of the above 5 answers did not address the real issue in the original question. In fact, the solution to the original question was resolved by Duvan's comment immediately after his question: have to install an application (e.g. CRM) after a Odoo installation.