@Alekjandar It appears another customer had this same problem last month and received an answer and verified the answer on Stack Overflow.
You need to create an Email Communication Services (ECS) resource and connect it to your Azure Communication Services (ACS) resource before start sending emails. Please have a look at below documentations:
Credit goes to Ebraheem
Can you please try out Ebraheem's suggestions to see if they also work for you? Let us know the results or if we need to provide further assistance. Have a great weekend!