Unable to install plugin on subsite

Hi, I installed the User Synchronization plugin on my main site http://www.sanjhachulha.biz. I have a subsite which is business.sanjhachulha.biz (which is a separate word press installation). I tried to install the plugin on the subsite so as I can make the subdomain a subsite. But when I try to intstall the plugin on subsite, it shows me that I have reached my installation limit. Can you please help as how do I solve the issue.
Thanks