This is false positive AChecker throws for WordPress native search form (so, not a theme issue here). The label is set correctly in the form though, it’s just not set the way AChecker wants… You can read more about this issue and how to fix it in my knowledge base article.
Please note that AChecker is not a particularly good tool for accessibility testing, like I’ve mentioned previously. WordPress does not recommend it either (check https://make.wordpress.org/accessibility/handbook/which-tools-can-i-use/useful-tools/). For example, if you look at the AChecker credits, it seems the script is at least 7 years old (with their API falling back to 2009). All the other accessibility tools have more modern, constantly updated code.