No. The decoder normalises - and _ back to + and /, strips whitespace and line breaks, and restores missing = padding automatically. Both alphabets and any wrapping just work.
Why do I get "not valid UTF-8 text"?
Because the Base64 decoded successfully but the resulting bytes are not text — you have binary data such as an image, PDF or compressed archive. That is a successful decode of non-text content. Use the Base64 to Image tool if it is an image.
What makes Base64 invalid?
Characters outside the alphabet (A–Z, a–z, 0–9, +, /, = or the URL-safe variants), or a length that cannot be padded to a multiple of four. The most common cause is a truncated copy-paste that cut the string short.