is this not a thesis statement ?

Various approaches to prevent steroids in sports have not been successful because athletes are now using hard-to-detect growth hormones to enhance performance.

I mean I don't see how this isnt a argument

Can you see that it is merely a statement of fact? For an argument you need to "pick a fight." Now if you said "Clear X should NOT be the case......" someone will debate the other side = "Wrong, it SHOULD because......"

Sra

But I thought thesis statements cannot have "should"

you need to change your approach. For instance, Steroid use by athletes should not be grounds for denying competitive opportunities. Then you can use various arguments to support that.

Meaning .. ?

What is it exactly that you don't understand.

These two sites will explain what a thesis statement should do and how it is constructed.

http://www.unc.edu/depts/wcweb/handouts/thesis.html

http://owl.english.purdue.edu/owl/resource/545/01/

You are correct, the sentence you provided can be considered both a thesis statement and an argument.

A thesis statement presents the main idea or argument of an essay or research paper. In this case, the sentence you provided presents the main argument that various approaches to prevent steroids in sports have not been successful because athletes are now using hard-to-detect growth hormones to enhance performance.

An argument, on the other hand, is a claim supported by evidence or reasons. The sentence you provided makes an argument by stating that the reason why various approaches to prevent steroids in sports have not been successful is because athletes have shifted to using hard-to-detect growth hormones.

So, in summary, the sentence you provided can function as a thesis statement for a paper discussing the ineffectiveness of approaches to prevent steroids in sports, and it also presents an argument by providing a reason for this ineffectiveness.