This error is coming from either line 204 or line 295 of the file Profit_Options.xml in the addon
Tradeskill Profit.
I'm not familiar with XML nor do I understand this byte attribute. You can try to remove those byte parts and leave the rest of the fontstring line or uninstall the addon since it hasn't been updated for BfA.
I'm also under the impression that this addon has always generated this error, but until now, it wasn't a player-visible error. There's an xml log file that has cached errors quietly for years until some yahoo at Blizzard decided to throw them into player's faces as if they're real errors. They aren't. They're useless and harmless.