How to install erpnext version 12 on Debian 9/10?

Your clue is in the output log - read and try this Install script fails on Debian 9 [2019-07-29] - #3 by lasalesi

If that installs version 11 then check around, that is a popular question!

edit: Please add here a pointer to what install notes and command line you used - to specify that info will help others like you

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