What else other than computation is required?
I have no idea. Neither does anyone else. That doesn't change the fact that computation alone has been show to be insufficient. That's pretty well established.
You can never state that something is not sufficient just because no one has figured it out yet. Most likely 100% of all significant technological advances were preceded by people who tried and failed. Often the people who finally figure it out were among the people who had failed multiple times in the past.
To say something isn't possible you have to know a great deal about why it is not possible. For instance we know that silicon transistors cannot shrink indefinitely because of our knowledge of physical limitations such as the width of a silicon atom. We don't say that transistors cannot be 5 nm wide just because all attempts at making 5 nm wide transistors have failed; that would be stupid.
It could not possibly be established that computation alone cannot create general AI because we have not even approached computers with enough transistors to match the 100 trillion synapses in an average adult. Even once we reach that milestone it will take a large amount of research before people could claim it simply isn't possible, and even then there would be a good chance they are still wrong.
Realistically the only way to say it is impossible to create general AI with computation alone is to actually create a general AI by other means. Only then could we possibly understand the process enough to know for sure it couldn't be done another way. Although even in that case there is no guarantee someone won't come up with a more elegant and/or efficient way in the future which only uses computation.