company excludes liability coverage when written as a LLC

[Padma Mody]

I bought one SF for rental under my LLC

 and insurance company excluded liability coverage. Agent is saying that they can quote a personal umbrella policy that will cover liabilities for all my rentals. 

I don't know what I should do for insurance? Is everyone buy the umbrella policy? And for what the amount of policy I should buy? And why company is not offering liability coverage to LLC?