No shutoff valve for tankless hot outlet
The first thing I would do is check if a permit is required for this job. You can look in the plumbing code for your city, but it would probably be faster and easier to just call the inspections department and ask them (and then you'll be 100% sure). If the work required a permit, find out if the installer pulled one (and check your invoice to see if you were billed for it). Also confirm if it required an inspection (some work does, some doesn't).

If a permit was required but not pulled, get on the phone with someone at the company and figure out why. This needs to be done urgently and they should treat it as an urgent problem. Do not wait and see, do this immediately, while you still have the option to stop payment on the check (the absolute last option, but one you want to have available just in case). Press them hard and don't let up until they fix it.
If a permit was required and one was pulled, first call the manufacturer and explain the installation to them. Send some pictures in an email if necessary. They will clarify exactly what is recommended. Next, get in contact with someone at the inspections department that deals with plumbing. Explain your concerns and see what they have to say. Usually inspectors want to see things installed in accordance with manufacturers instructions, at a minimum. Any deviation will generally need to go above and beyond what is called for; leaving out a valve that is specified in the instructions doesn't do that.

At the end of the day, you need to comply with the inspector, and the best way to do that is to make sure the manufacturers instructions were followed. This will also be an issue down the road if you ever need warranty service on the unit, so get it sorted out ASAP.