Also spotted the following solution:

If you've go several OE versions installed on your Windows OS, you may need to change the pg_path variable of the .conf file of your Odoo server.

I saw a case where, with a v11 and v12 install, the v12 had pg_path = None, which made it impossible for Odoo to guess that postgresql's bin location

Hello,

You can try take backup from here

Log into the database as an administrator through postgre Admin III and backup your database from there.

May be this will help you.

Thanks,

Shamji

You could config the exact location of your pg_path. This could be done in your config file.

pg_path =C:\Program Files\PostgreSQL\12\bin

this worked for me.

But 

This path may vary in your system

So check the file path in your Program Files >>PostgreSQL  and copy the path of bin folder

Check this out: https://github.com/Yenthe666/InstallScript/issues/178

If you are on windows consider adding the  'C:\Program Files\PostgreSQL\10\bin' to your PATH environment variables and restart the odoo server. After that, you should be able to backup from the UI.