Note that all transactions need to be associated with a financial account. It can be a synced account or a manual account. If you want to manage cash transactions in Monarch, then you have 2 options.
Option 1 - Manual account
Setup a manual account and name it something like "Cash Wallet". Learn more about how Manual accounts work. With a manual account you can track all your cash transactions manually.
Option 2 - Split transactions
Split existing ATM transactions into multiple transactions. This works best if you already have an account synced to Monarch that shows the ATM withdrawals. Learn more about how to Split transactions.