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There is a a way to Easy Install version-11 on Ubuntu 18.0.4LTS

Dears,
if you would like to install version-11, please modify the install.py after downloading it on your server.
Search for the string 12 in the file and change them to 11 except the 12 in the operating system check routine. There should be only one 12 in the file after you modify it.

After that, you are good to go with the Easy Install command. Do not include the branch related operators in your command and you will be good to go if you have installed the prerequisites:

This will solve your upgrade to version-12 woes and get back you to a healthy verison-11 again where you can restore your backups. Upgrading to version-12 from that point is not tested yet. Please let me know if you have walked this path.

python install.py --develop --version 11 --user [frappe-user]

u can also install v12 and then switch to v11