I want to update my MacBook version to 10.14

The age of the machine matters.

For the MacBook Air, it must be mid-2012 or later to upgrade to Mojave. 2011 and early 2012 will not work.

If you are using the correct version, make sure all the other system requirements are met:

https://discussions.apple.com/docs/DOC-13571

Be sure to use only Safari to do the Download - other browsers will not function