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As it says at the experimental package list,

These are EXPERIMENTAL! They probably won't work at all for you! Use at your own risk!

However, in your case it seems that there is some other issue. It should look like this:

$ ./sage -i asymptote
Attempting to download package asymptote
>>> Checking online list of optional packages.
[.]
>>> Checking online list of experimental packages.
[.]
>>> Found asymptote-1.29

WARNING: you are about to download and install an experimental package.
This probably won't work at all for you! There is no guarantee that it
has ever been built correctly! Use at your own risk!
Are you sure you want to continue [Y/n]? y

That an exception has been thrown before this indicates you may want to check your permissions or internet connectivity.