Dear Bilguun,

please stop spamming the mailing list with every single error
message you get. This is not a pastebin after all. Don't get
me wrong, I am not trying to turn you out-of-doors,
but you should be able to resolve such issues yourself.

In this particular case it's obvious, just read your own message:

> There is no such command.
> Error occurred during reading the below line:
> band GSM900

it already contains half of answer. Most likely, your configuration file
missing the proper alignment, have a look at the examples:

https://git.osmocom.org/osmo-bts/tree/doc/examples/trx

> Im following this article.
> https://bastienbaranoff.wordpress.com/2018/08/10/gsm-base-station-with-two-osmocom-bb-compatible-phones-on-kali-rolling/

This article might be outdated. I recommend to follow our own wiki.
If you are going to run a base station, you should really understand
what you're doing, but not just copy-pasting from random articles.

There is a tutorial about CalypsoBTS, read it carefully:

https://osmocom.org/projects/baseband/wiki/CalypsoBTS

Thanks for your understanding.

With best regards,
Vadim Yanitskiy.