Hi John, it is a great question. WACC and CAPM are slightly different concepts. Both do tell us the cost of capital, but in different ways. CAPM (Capital Asset Pricing Model) states that an asset's expected return depends on the asset's risk through the asset beta (or market risk) and not through the asset's specific risk. CAPM can be used to estimate the return on assets if we know the beta of the underlying asset.
WACC is a cost of capital calculated using the returns expected on each of debt and equity, multiplied by their proportional weights in the capital structure. In the case of no taxes, both WACC and return on assets through CAPM both are equal. However, WACC changes once we introduce taxes. This is because there is usually a tax break on interest payments for debt.
In a nutshell, CAPM and WACC can be used to calculate cost of capital for a company and produce same result when there are no taxes.