Macbook Air 2015 High Sierra cannot merge free space partitions
The simple fix is:


Back everything up (apps and files and settings and possibly keychain passwords if you don’t have them written down / recorded elsewhere)
Boot to internet recovery
Erase and start with one partition


Using the command line to merging containers requires you to clearly understand quite a few items and processes and concepts and one mistake ruins the data. It’s like several passes on a trapeze at the circus, without a net. It takes months of experience to make it look effortless and years of experience to always succeed. 

Here’s your guide to easy and the site will be here as you gain storage knowledge. 


https://support.apple.com/en-us/HT201065