Required-field and null-value errors in flows
Key Takeaway
Make sure every path in a flow supplies a value for each required field, and check for blanks before the flow uses a value.
A flow fails with a REQUIRED_FIELD_MISSING error when it creates or updates a Salesforce record without providing values for all fields marked as required. The flow fails at run time, not while you are building it. A flow also fails when it uses a value that turns out to be blank (null) in a place that needs one. App builders prevent these errors by finding required fields before they build, making sure every path in the flow supplies values, and checking for blanks with Decision elements. When an error still happens, fault paths, Debug mode, and flow error emails show which element and field caused it.
What can make a field required
Requiredness doesn't come only from the field definition. Different record types can require different fields. Validation rules can also make a field conditionally required. Because of this, a flow can work for one record type and fail for another.
For example, a contact flow might need three fields for three different reasons. Last Name is a standard required field. A validation rule requires Email. A record type requires Account.