Pascal Delphi - Undeclared Identifier

I'm using the Synapse library in a Delphi project to do some networking.

When I try and use one of the Types that are defined in the external units, i.e: 'TTCPBlockSocket', it has the red underline and says "Undeclared Identifier 'TTCPBlockSocket'".

The files are all in the correct folders and the 'uses' statement can find them and shows no errors.

The strange thing is that I've had an identical setup in another project and there have been no errors in that project.

As a note: I can't install/edit the software/settings, so I can't add any fixes.

This is a well-known bug in Error Insight. It has existed since Error Insight was first introduced in Delphi 2005 or 2006, and still exists today in Delphi XE5.

It's caused by a different compiler being used for Error Insight, apparently, that doesn't have access to the same symbol set used by the Code Insight compiler (the one that helps you find symbols in the Code Editor) or the command-line compiler (the one used to actually compile your code for an application or package). It therefore only uses files that are actually referenced in the project (.dpr) file to locate symbols, and since the majority of files in the uses clause in your code aren't in the .dpr, it can't find them.

There are two fixes (one that is very easy, and one that works but is a pain in the backside):

(The easy one). Turn off Error Insight totally, in Tools->Options, the Editor Options section, Code Insight; just uncheck the box for Error Insight. I prefer this one because Error Insight doesn't work properly anyway, and it avoids the annoyance of having to use the other option every 10 minutes. This is the first thing I do when I install a new version of the IDE and see the red underlines.
(The pain way). Use Project->Add to project for every unit that contains one of the underlined symbols. This adds a reference to every unit to the project .dpr file, which causes it to increase in size drastically. It's a pain because you typically have to do that for every single unit (in my experience, including those that are part of the standard VCL/RTL) that hasn't already been added, and it very quickly becomes irritating. Error Insight doesn't tell you anything that a quick Ctrl+F9 won't anyway, IMO.