Could he not have just morphed into another creature then demorphed back to Andalite?
In book 10, The Android, it says:
< [...] He even reattached my arm. >
I saw a scar on Ax's left arm.
Could someone explain this, please? Or does he fix it later on? (I'm only on book 10, so no spoilers please.)
From an in-universe perspective, the best answer is "morphing is just like that sometimes". There are numerous cases where morphing technology is shown contradicting what the Animorphs are told about it, usually in small ways. Of course, in some of those cases, the real-world answer is that K.A. Applegate and/or her ghostwriters made mistakes, miscommunicated, or just came up with better ideas after the fact. However, it appears that some of this ambiguity is in-universe as well.
Morphing is a relatively new technology, probably about an Andalite generation old, or maybe less. (Full, spoiler-y discussion of this point can be found in this answer.) Andalites consider morphing itself more of an art than a science, even to the point of having staged performances. Prodigies at morphing can push the limits of what's usually considered possible.
Without getting into too many details, there are both situations where morphing repairs damage like you're suggesting it would, and situations where it doesn't. I suspect there will never be a definitive in-universe answer about why.
Erek saved them all just as they were teetering on the edge of blacking out and dying horribly. With his speed and the obvious life-threatening injuries in front of him, he likely just did what he could as fast as he could, unaware that things like missing limbs are not as lethal to morphers as they are normally.
Erek was an ally, but being an ally doesn't mean being 100% aware of all of the limitations of your special powers.
