Fonts are not always free. People make a living out of font design. My first encounter was when I designed and implemented a series of barcode reader installations for a customer. The configuration guide I wrote required the barcode font (3of9 I think).
Which we had to buy.
I was reminded of this when our support team reported the following problem.
Multiple users report a
“FooBar font missing, default font substituted”
Indeed it is missing, the FooBar font is not part of Adobe Illustrator, and that’s what the error message is trying to tell you, if you read it.