Is there a way to save a gif in photoshop without the 'save for web (legacy)' feature?

Do you have permissions to save to the folder you are using? That's where the error message comes up.

"Adobe Save for Web Error
Could not complete this operation. A write permissons error has occured."

I believe the error originates from the operating system rather than Photoshop. Where are you attempting to save the file? Do you encounter the error every time you try to save a file, only when saving a GIF, or just when saving that specific file?

Are you trying to save a frame animation, or just a flat single layer, or maybe one with transparency?

If the answer is the latter, and you are not saving an animated GIF, then you can use Save As.

This will bring up this simple dialog.  Note I only have two colours as I was testing this for the screen shot

With a full colour image, the max number is 256, like with an Indext Color mode.

However, you shouldn't be seeing that error message, so can you give us a bit more information?

What are you saving, an how are you trying to save it?