How to input fractions such as a half, quarter etc into text and why is my 'Substitutions' option greyed-out on my Macbook Air

All text input on a Mac revolves around fonts. There are a variety of native macOS fonts that include fractions. To see them use Font Book. It's in your Mac's Applications folder.

These are from American Typewriter Regular:

¼

½

¾

For a quick way to insert a fraction I suggest using "text replacements". To learn about it read Replace text and punctuation in documents on Mac. Create a key combination that you want to use for each fraction. For the above example I used f1/2 to substitute the fraction for 1/2... etc.

Here are a couple of Apple Support documents to get you started:

New to Mac - Apple Support

Mac tips for Windows switchers - Apple Support

You may have to add the Digits - All category to the Emoji & Symbols panel (^⌘spacebar). In the upper left corner of this palette, there is a gear symbol with Customize List… . A categories sheet will descend, and you will find Digits - All in the first few categories. Once selected, it will appear on the left category panel, and you will find fractions within this category, or the means to construct fractions.

You can always type in 1/2 2/3 7/8 27/32 etc. if you can't find them.

Your Mac includes the Font Book app. If you cannot find it, then that is another problem that needs to be solved. You don't need to buy anything else.

Kindly refer to Fonts included with macOS Mojave including instructions for installing and removing fonts.

For instructions: Font Book 

I have also found that in order to use text replacement you have to type in the word you want to replace and the word you want to replace it with.

That's not true.

If you need help using Font Book, or text replacements, please refer to the Help menus available on your Mac, or ask.

Perhaps as I am a compete Apple Mac beginner I am looking in the wrong place. I checked the Apps menu at the bottom of the screen with the big 'A' on it against blue background. Am I looking in the right place?

No... as I wrote it is in your Mac's Applications folder.

If you are a complete Mac beginner I suggest reading the Apple Support links I provided. To do that, click on the blue text:

New to Mac - Apple Support

Mac tips for Windows switchers - Apple Support

A Safari window or tab will open and those documents will load.

FWIW, everyone has Font Book. It's been installed with every version of the Mac OS since 2003 and OS X Panther.

See…

They are in the character viewer.

Ctr-cmd-space

search for fraction and you should see them.

Text replacement is the best way to do it. The text you type is the unformed fraction. Use the character-based fraction as the substitution.

1/8 is what you type, and ⅛ is what is substituted

https://support.apple.com/en-us/HT201586