What should I do if the workplace certificate for a French tourist visa does not have company letterhead?

You can make it yourself. Write the company name in the header, then make it larger and bolder; or attach an icon such as the company's trademark in the header, and then write the company name.

The letterhead is one of the factors that proves that the format is correct, but what the visa officer really checks is the official seal and contact information, especially when calling, to ensure that the information is consistent with the work certificate, business license, etc.