If you want to be able to issue more units of an asset after it is created, you need to set it as open when you create it. For example you could pass this object to the issue command in place of the name asset1:
{"name":"asset1","open":true}
If an asset is open, you can issue more units using the issuemore command.
All this is documented in the 'Asset management' section of the API guide:
https://www.multichain.com/developers/json-rpc-api/