I can’t choose the currency when adding a transaction
Transactions use the default currency, you can set it in Settings, tap the cob icon on the tab bar and choose you default currency.
Why I cannot delete a crypto Buy transaction, it says something about being linked?
If you added an exchange transaction, it is not possible to remove the buy transaction from which the coins where then subsequentially exchanged.
You have to first remove the exchange transaction and then you will be able to remove the buy transaction.