It would help to see the relationship linking table 1 to the portal to table 2.
Something is not set up quite right with either the relationship, or the details of your portal set up or both.
The error message is most likely indicating that a field in table 1 that is defined as a match field for this relationship does not have a valid value, most likely, this field is empty at this time.
If you have this type of relationship for your portal:
Table1::primarykey = Table2::Foreignkey
where primarykey is an auto-entered serial number.
You can double click the line linking these two Table Occurrences and select the "allow creation of records via this relationship" for Table 2. Then, simply entering data in the bottom blank row of the portal will create a new related record in Table 2, auto-entering the current value of primarykey into the Foreignkey field of this new record in table 2.