I'm not sure if it's an instance related feature. I think it's more client related. On the original mastodon app for Android, I couldn't find it; on Fedilab, it's there (on both instances I'm using). Didn't bother to look while using tusky.
But it seems instances might impose a time limit on editing ("you can only edit it for x minutes after creation"). Maybe.
Have you tried a different client yet?
@purplequeennl
@ajsadauskas
I've just discovered that editing was introduced with version 3.5, so instances running older versions are out of luck. But writing exchange seems to be running 3.5.3, so the version is not a problem, it seems.