it should clear up once it compiled. VS has a lot of bad "error" underlines lately. I ignore them and fix the ones the compiler finds.
are they C headers, eg math.h, or other standard ones with .h on them, by any chance?
IntelliSense (the red lines that show up, not compiler errors) keeps a cache so it isn't constantly re-parsing your files over and over again.
Sometimes this can be buggy or not update properly. I think deleting the hidden .vs folder in your solution will also delete the IntelliSense cache, so you could try that.
Yeah, the last couple of updates to VS 2019 has made Intellisense more than a bit twitchy.
Deleting the hidden .vs folder removes a lot of configuration settings VS keeps track of. Including if the project/solution defaults to debug or release configuration and x86 or x64. It also nukes any user-defined configuration settings.
I've found the easiest way to give Intellisense a kick in the bum is to perform a compile of a source file (CTRL +F7) when the source listing is focused.
2nd easiest is close the project/solution and reopen it.
3rd easiest is to just give VS a bit of time (10-15 seconds) to let Intellisense stop being Bevis-ish.
If the rhombus in the bottom left of the VS window before Ready displays scrolling lines, then VS is performing background processing - including intellisense. Only when the rhombus is blank have the background processing completed.