ENVI's vegetation index calculator must know the wavelengths of the bands in order to determine which indices can be calculated. You can add in the wavelength information using File ->Edit ENVI Header ->Edit Attributes ->Wavelengths.

Thanks, But now there is the following error "No band combinations Can Be Found That Match Those required to calculate any available vegetation index". Using the Landsat-5 TM BANDS, 1,2, 3,4. Need to enter the wavelength of these bands

If the error is 'Need to enter the wavelength of these bands' then it sounds like the wavelengths are still not defined in the header. In any case, the VI calculator is intended for images that have many bands. Since you only have 4 band landsat data, the only indices you will be able to calculate is NDVI or a simple ratio. These can also be calculated using Transform ->NDVI or Transform ->Band Ratios.

If you can help me with an example of the wavelength, because I think I'm wrong when entering values. File -> Edit ENVI Header -> Edit Attributes -> wavelength - edit _item selected?. I'm working with 6-band LANDSAT -5 Thanks