Jos automaatti on rikki, menen pankkiin nostamaan käteistä.

Because Finnish normally separates an initial subordinate clause from the main clause with a comma. Here Jos automaatti on rikki is the conditional subordinate clause, and menen pankkiin... is the main clause.

Jos introduces a condition clause (like English if). It usually means if, but depending on context it can also resemble whether (e.g. En tiedä, jos hän tulee in some colloquial varieties), though in standard Finnish that whether meaning is more often expressed with tuleeko / tuleekö-type structures.

In Finnish, automaatti can mean an ATM in context, especially when talking about withdrawing cash. More explicit options include pankkiautomaatti (bank/ATM machine) or OTTO-automaatti (a common ATM brand/network in Finland).

Finnish commonly expresses be broken as olla rikki (to be in a broken state).

• on = is (3rd person singular of olla)
• rikki is a predicative word meaning broken / out of order.
  You can also see rikki with other verbs, e.g. mennä rikki = to break (happen to break).

Finnish verb endings usually show the person, so the subject pronoun is often omitted. menen already means I go.
You can add minä for emphasis/contrast: Jos automaatti on rikki, minä menen... (e.g. I will go, as opposed to someone else).

menen is 1st person singular (I go). menee is 3rd person singular (he/she/it goes).

• minä menen
• hän/se menee

pankkiin is the illative case, expressing movement into something: to/into the bank.
Base form: pankki → illative: pankkiin.

Yes, but it changes the nuance:

• pankkiin = into the bank (inside the building)
• pankille (allative) = to the bank (often “to the bank” as a destination/area; can imply you go there but not necessarily inside) In everyday speech, pankkiin is very natural if you mean going in to do banking.

nostamaan is the 3rd infinitive illative form of nostaa. It often expresses purpose, similar to to (do something) in English:

• menen pankkiin nostamaan käteistä = I go to the bank to withdraw cash
  Literally: I go into the bank (for the purpose of) withdrawing cash.

Core meaning of nostaa is to lift/raise, but it’s also the standard verb for withdrawing money:

• nostaa rahaa = withdraw money
• nostaa käteistä = withdraw cash
  So it’s a fixed, very common banking usage.

käteistä is the partitive singular of käteinen (here meaning cash). Partitive is common because: 1) cash is treated like an uncountable substance, and/or
2) you’re not specifying an exact total amount in the sentence.
If you specify an amount, you often use something like: nostan 50 euroa käteistä.

As written, it’s a normal “real/open” condition: If the ATM is broken, I go/will go to the bank... (a plan or habitual response).
For a more hypothetical/less certain meaning, Finnish often uses the conditional mood:

• Jos automaatti olisi rikki, menisin pankkiin nostamaan käteistä.
  = If the ATM were broken, I would go to the bank to withdraw cash.