Cannot Open Rectangular Grid Tool in Illustrator

I'll bet what you're seeing is this:

This isn't where you pick tools; it's the window used for editing the contents of the toolbar. If a tool is greyed out, you already have it in the toolbar.

By default, it is located under the line tool. Click and hold to reveal it:

As Jeff suggested, there are other good techniques. It would partly depend on the logo, whether it is a bitmap or vector. If it is bitmap, another technque would be Object > Create Object Mosaic. 

In any case, even if you decide on another technique, I'm sure that you would want to get your Rectangular Grid tool back in working order.

A "pixel-like" logo might be done more easily with Split Into Grid. Object > Path > Split into Grid. Each object in the resulting Grid is a separate object.

You could try resetting your workspace (Windows menu)

Thanks for telling what version of Illustrator you are using.

But again, can you also show a screenshot that may better explain what your issue is about?

Since people are now calling everything a "tool": what exactly are we talking about? The actual tool in the toolbar or something else?

I'm using Illustrator 2020 (the latest version.  

Try resetting preferences. https://helpx.adobe.com/illustrator/using/setting-preferences.html

Which version of Illustrator are you using on what operating system?

Also can you post a screenshot that shows the greyed out tool?