Image -> Adjustments -> Desaturate is greyed out for me.

[Hee-Dee5ECC]

It is 8 Bits/Channel. I tried converting it to a smart object too. 

[Chuck Uebele]

Can you post a screen shot? What color space are you using? 

[Hee-Dee5ECC]

[Michael Bullo]

Do you have something like an Adjustment Layer or a Mask selected within the Layers panel?

[Hee-Dee5ECC]

I don't, I just have the layer selected.

[Junyyy · Accepted solution]

You might have to rasterize the layer. It worked for me.

[Bojan Živković]

Your screenshot is showing us exactly what is happening when Smart Object layer is selected in the Layers panel.

Try to select pixel based layer. If nothing helps try with preference reset what is general advice for all things weird in Photoshop https://helpx.adobe.com/photoshop/using/preferences.html

[Ta2776955754ai]

Hi! I Went to Properties=>Convert to Layer. Now i can use the desaturate function. 

[Stephen Marsh]

If you wished to retain the Smart Object, you would need to edit the SO and then add a non-destructive Hue/Saturation layer and adjust the saturation and then save the SO file to update the original layer.