It appears the constraint is related to the generated column:

https://docs.databricks.com/delta/generated-columns.html

"For example, you can automatically generate a date column (for partitioning the table by date) from the timestamp column; any writes into the table need only specify the data for the timestamp column. However, if you explicitly provide values for them, the values must satisfy the constraint (<value> <=> <generation expression>) IS TRUE or the write will fail with an error."