Missing Scheme

Hello,

When I use the https://validator.w3.org/nu/ for makanindonesia.nl or makancatering.nl I get the message: io missing scheme

What does io missing scheme mean?
How can I resolve it?

Thank you, kind regards,

Makan Indonesia BV
Catering ⋅ Traiteur ⋅ Restaurantbezorgservice