Automations turn off after an error

I'm not sure if this is exactly what you're going for, but I've used a pretty simple solution for something similar in the past. If you create a relationship between Table A (child) and Table B (parent) using your match field as the reference, you can create a Formula - Checkbox field in Table B called Parent Exists? or something along those lines. The formula in that checkbox is simply "true."

Now create a lookup field in your relationship, where Table A wants to pull down that Parent Exists? field. If there is a match in Table B already, that lookup with be populated with a checkbox. If there is not yet a match in Table B, and therefore the record in Table A is an orphan, the lookup will be blank.

You can filter your automation to only run when the lookup checkbox is Table A is NOT checked, and it will not try to create a duplicate record in Table B.

If your intent is to get an error message instead in Table A, you can probably use form rules or some more complicated code to get that to happen right when you enter the data and it realizes that box is checked. Alternately, if you want to force the Table A value to be unique, you can create a Formula - Text field that checks if the Parent Exists? box is checked, and if so, adds some unique identifier.