You may, though you may have to be careful about that you say. Providing information about alternative medicine is legal in the US. You can read this article which addresses unapproved medications and therapies from a medical policy perspective, touching lightly on legislation. There are restrictions, enforced by the FDA and the FTC, on what you can sell and claim for your products, in case you sell dietary supplements or are in some other way making a business of purportedly curing people. Here is a starter page about FTC regulation of health claims. Here, for example, are some actions that the FTC took against companies for unproven CBD claims, such as an action against Bionatrol, with many kinds of purportedly false claims made "In connection with the advertising, promotion, offering for sale, sale, or distribution of CBD Products". It's not the claims that are illegal, it's making the claims in commerce that's illegal. The FDA regulates drugs and devices, and this page divides the FDA regulations into functional types such as "drugs" and "medical devices". It would be illegal to sell a "brain ray machine" that purports to cure cancer, but it would not be illegal to describe how to build one.
There are a number of DIY treatments available on the internet, for removing ticks, slivers, for bandaging scrapes and so on, none of which have or require government approval (in the US). In some cases, such a website might infringe copyright or a patent, so that would be a way in which the website could be illegal (Four Thieves Vinegar). Without any further information on what such a website is saying, it's hard to be sure but this gives you the general limits on the legality of such a site.