Hello,
This warning is misleading in version 5.3.3 and prior. We have improved it in v5.4. Based on the Log file that you attached, I see that “PFAS_SOURCES” is the name you have assigned to a family of emission streams. I believe this note is telling you that one or more of those emission streams could not be found. However, it probably found some of them and correctly applied the emissions. You will have to visually inspect the log file output after the content in the file you’ve sent to be sure. This should include the sections summarizing the species-specific scale factors applied for every stream.
Best,
Ben