I'm not surprised they struck it down, but I guess the jaded side of me is surprised they allowed him a different avenue to do the relief instead of making the whole concept illegal.
Also, there was talk from the Republicans about try to force everyone to pay back the interest we would have been paying this entire time. Somewhat surprised they didn't agree to that as well.
There's nothing formal stopping the SC from doing anything, but courts are generally limited to ruling on the controversy in front of them in as narrow a way as practically possible. I haven't read any analysis on this ruling, but just from the little I have seen, it looks like they ruled that the HEROES Act didn't grant the federal government the ability to forgive the loans in the way they were attempting.
Biden could try using an authority from a different law or creating a different set of rules by which the loans may be forgiven.
My non-lawyer prediction is that if Biden tries again, the SC will find a new reason to stop it and will make a bigger ruling that takes more power away from federal agencies to make decisions. They've already been doing this with environmental and health decisions, and I'm sure other agencies have been impacted too.