Go - error: panic: runtime error: invalid memory address or nil pointer dereference-JZTXT

Then looking at this code:

res, err := client.Do(req)
defer res.Body.Close()

if err != nil {
    return nil, err
}

I'm guessing that err is not nil. You're accessing the .Close() method on res.Body before you check for the err.

The defer only defers the function call. The field and method are accessed immediately.

So instead, try checking the error immediately.

res, err := client.Do(req)

if err != nil {
    return nil, err
}
defer res.Body.Close()