hi @victoryau and welcome to WF Forum. The issue is caused by your div-manga-home that on mobile viewport covering (is above) your form element. The easiest way is to set z-index on form element. But the issue, why form element is covered should be revisited anyway in first place . When this will be fixed z-index would not be necessary.
BTW: It is good practice to use search input field on this forum to look for solutions before creating a new request. A similar issue was requested and solved a few hours ago and I believe there are other solved requests with identical issue.