If you have have across this error: unable to complete request error then this solution could be what you just need to do to solve it.

1) Flush your Magento cache storage
First point of call, should be flushing your magento cache storage.

System -> Cache Management -> Flush cache storage

Now try to assign the attributes again.
If that fails – disable cache management all together for the duration of the assignment process

System -> Cache Management -> Disable All Cache

You may also want to delete the cache files all together should you still have problems. /var/cache/