Users don't have admin permission so you can't use sudo command

Hello Gonzalo de la Serna, 

You can try this command.

sudo chown -R root:odoo /usr/lib/python3/dist-packages/odoo/customaddons/
sudo find /usr/lib/python3/dist-packages/odoo/customaddons/ -type d -exec chmod 755 {} \;

or

sudo chmod 755 /opt/customaddons

I hope this will help you.

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari