What can I do if an insurance company’s offer is not sufficient to replace my totalled vehicle?

Is this your insurer (e.g. your own collision coverage) or another party's insurer (e.g. the liabiltity insurance of a driver who hit your car)?

Your collision coverage: check what the policy says--the insurer has to pay as per the policy's terms. If the policy obligates them to replace your vehicle with a comparable one, that's what they'd have to do, and if they don't, you could sue your own insurer to enforce its obligations. However, most policies only obligate  the insurer to pay the then-current value  (the "blue book" value) of the car at the time it was totalled. This is almost always less than the cost to replace wiith something comparable; however, since that's all the insurer is legally (contractually; the policy is a contract) required to pay, that's all you can get. If they're not even offered the then-current value of your car, you could again sue to force them to at least pay its value.

Other driver's liability: the other driver, if at fault, would have to pay the then-current value, not replacement value, though would also potentially be responsible for other direct, out-of-pocket costs (e.g. towing; renting a replacement until you can buy a new car; etc.). If the insurer has made you a settlement offer and you are not satisfied with it, you could sue the other driver. To prevail, you'd have to prove he/she was at fault, show that you were not at fault, too, and also prove your losses or costs.