Skills Aren't Binary
Skills aren't binary. You don't just learn how to do something. You aren't either an expert or completely ignorant. There are many levels to most skills. And so when you start learning, you begin a journey in developing yourself.
It takes a lot of time to get good at something. Any fundamental skill is something you just can't afford putting off the development of. If you aren't jumping in to learn something that you should be, you aren't just a little bit behind... you might actually be behind your peers by all of the time it takes to gain adequate experience, which might be several projects, maybe even years. Those that have already started the journey of actively learning are off and running and those that put it off are still standing still at the starting line.
Take a fundamental like unit testing as an example. There aren't many software development skills that are almost universally agreed upon as essential and beneficial, and yet there still exist leagues of developers that continue to procrastinate and put off the investment into learning and practice.
Learning the skill of testing is a progression through understanding some absolute basics of setting up a project properly, learning and rethinking structure and code to make it more testable and maintainable, different strategies for isolating components for testing, learning how to structure and maintain the tests themselves, becoming effective with new workflows and methodologies, maybe some automation tricks, and a lot of time learning how to distinguish between tests that provide value and those that don't.
Testing isn't something you just pick up and do, it's something that you begin learning that requires an investment in time to get good at. It also fundamentally changes the way you think about and design software and is essential to maturing as a developer and learning how to create quality work.
For fundamentals, there's absolutely no excuse to put off learning if we ever want to be amazing.
If you haven't started the journey, you're probably just falling further and further behind... whatever are you waiting for?