"Can't add script behaviour AssemblyInfo.cs"

so i already read a couple other posts of this same problem and a few similar ones: no dice. i keep getting this error when trying to add a script to an object, ANY script, even ones that are freshly created by unity and unedited. i had a similar problem yesterday but the error it kept throwing at me was "Can't add script behaviour VisualContainerAsset" but it was the same effect, wouldn't let me add any script at all. no clue what to do, please help. why is unity being such a pain?