What can be the remedy for not having insurance in the past?

Were there any claims made for that time period? Are you the type of business where claims would be made? This may be a case of no harm, no foul. However, I suggest you talk with a local real estate attorney to go over your lease and discuss the situation. An argument can be made that the failure of the landlord to insist on insurance at the time was a waiver by the landlord and he cannot insist on it now. But an attorney can make that argument best.

Your landlord will probably not get an order of eviction if you cure.. so get a current insurance policy and cure! Most likely he will not be able to evict you if you do that. If he tries, argue that he waived the past breaches by not timely demanding an insurance policy and that you got one once he made the demand!

The failure to procure insurance on a commercial lease is typically a curable default. The only exception would be if the lease itself states that the failure to maintain insurance is a non-curable default. Curing the default might include the payment of claims that would've been covered under insurance if there was insurable harm that resulted in the disclosure that the insurance was missing. Again, whether such an amount justified an eviction, would depend on whether the lease characterized damages resulting from the failure to ensure as being additional rent payable by the tenant.

Technically it is not curable, since I don't think you will be able to bind coverage for 3 years retroactive, but I also don't think it is a material breach since no liability claims have been made and I assume the landlord did not make prior demands for coverage. So you have a reasonable defense to the eviction. Get insurance now and fight the eviction suit when filed.