This is a memory error; essentially you do not have enough memory to open the SILVA classifier on your computer.
This is low for the SILVA classifier — it will often take up to 32GB+ if left to its own devices!
There's a couple of things you might be able to do.
First, you can use the --p-reads-per-batch parameter (e.g., set to 1000 or 2000) to reduce the number of reads classified at a time. However, it looks like you do not have enough memory to load the SILVA classifier, not the reads, so this might not help.
The other option is to use the greengenes classifier, which is much smaller and hence less memory-intensive to load.
the only other option will be to find a more powerful machine to run this on, e.g., an computing cluster, AWS.
Good luck!