Why is this C code section flagged as bad? [closed]

Any decent linter will flag numerous problems with this code. Here’s what the one provided with Atom editor found, plus what I eyeballed.

int lookup_personal_info(char *first_name, void *last_name, uint8_t (*sin)[9], const struct **info_out)

last_name is declared void * but used as a char *
const struct **info_out declares an anonymous struct. It should be personal_info **info_out
The function returns a bool not an int.
sin is never used except to check it’s passed in.

    if (!first_name || !last_name || !sin)
        return false;

Passing in a null pointer is probably a bug by the caller. This silently ignores that bug making it difficult to find. It should raise an error.
It forgets to check info_out.

    char *initials[3] = { first_name[0], last_name[1] };

This is an array of character pointers being initialized with char. It should be char initials[3].
It is not null terminated. Maybe it doesn’t have to be, but I wouldn’t risk that.
last_name is declared void * yet used as a char *.
It’s using the first character of first_name as an initial, that makes sense, but why the second… thing… in last_name?

    personal_info *data = malloc(sizeof(struct personal_info));

struct personal_info is not a type, the type is personal_info.

    *info_out = (personal_info *) data;

info_out was incorrectly declared as an anonymous struct. It should be personal_info **info_out. Then no type cast is necessary.

    bool is_ok = database_lookup(initials, &data);
    if (is_ok)
        goto fail;

The check is backwards.

The use of goto is justified to ensure proper cleanup on error. It’s probably included in the example as a red herring.

fail:
    data = NULL;
    free(data);

    return true;

free is called after nulling out data leaking memory.
There’s no need to set data to NULL, it’s local to the function which will shortly exit.
Both failure and success return true.