QGIS 3.4 Spatial Join error message interpretation

You have several options.

The mas recommendable is that you use the tools of topology to locate the entity with problems.

Then you can activate it by clicking on the

Then click on the settings button, then configure the first rule, to do this select your layer, in the different options you will find the

"must not have invalid geometry"

, select it and add the rule with the green cross. Clic Ok

Before continuing I recommend you to assign to your layer a single symbol with a color that contrasts with the red, it can be yellow, blue, green.

Finally click on the check button as seen in the image, you highlight in red the invalid entities and you number them in the dialog box, including the ID.

Other options:

The ID is the sequential number as the entities are created you can add it to the table of attributes

Open the attribute table, activate the field calculator, add a new field, search for the expression "record and attributes" and enter $id