Are you doing this manually? If so; can you create a unique-list of PO#s and first import the PO# into the parent-PO table? The problem with creating the record in the Parent table without something "pinging" Quick Base is that your import is a multi-record action. Automations will not read each record individually and evaluate whether the parent exists to create the missing parent.
If you are not doing your imports manually; then I recommend a google-script solution that pings your table periodically to generate a list of unique PO#s. Then using that unique list, the google script can import your PO#s to the POs table; and anything that does not already exist will get created.
