"The document cannot be saved as jpeg without losing information" when saving .psd

To be absolutely clear:

The jpeg file format specification does not support layers, 16 bit depth, transparency or alpha channels. If the document has any of these properties, it cannot be saved as jpeg without removing these properties.

I wish Adobe had just stayed with the pre-CS5 policy: if the file does not comply with the jpeg specification, it cannot be saved as jpeg. Full stop. Clear and logical and transparent.

Instead they have introduced one workaround after another, all of which ultimately just confuse the issue and confuse users who don't recognize these fundamental limitations in the jpeg file format.

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@elektricstudio wrote:

There's another post about this but the answers make no sense.

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The other topic is here and make sense to me:

https://community.adobe.com/t5/photoshop-ecosystem-bugs/problem-saving-a-quot-single-layer-quot-document-as-psd/idi-p/15451062

What might be causing the issue? It's the way jpg's work.

Can it be safely ignored? Yes.  Kinds of depends if using jpg makes you all warm and fuzzy inside- use what you prefer.

@elektricstudio Essentially, when you open a JPEG, it's a flattened, single-layer image. But as soon as you add a new layer, an adjustment layer, a text layer, or a mask, you're creating information that a standard JPEG file cannot store. So, you can't save it as a JPEG. To save it as a layer file, you need to save it as a PSD. The warning is Photoshop's way of telling you, "Hey, you've added layers and other important stuff that will disappear if you save this as a JPEG. Are you sure you want to do that?" If you save the file as a JPEG, Photoshop will "flatten" the image, combining all your layers into a single background layer. You will lose the ability to go back and edit individual layers, text, or masks. The best option is to save it as a Photoshop file (.psd). This format is specifically designed to retain all your layers, adjustments, and other editing information.

If by chance, need a JPEG for a specific purpose (like uploading to a website or sending to someone), you should do the following:

• Save your master file as a .psd. This is your primary working file.
• Then use the  "Save As" or "Export As" function to create a separate JPEG file from your master PSD. When you do this, you'll be given the option to flatten the image, and you can adjust the compression quality. You should always keep the original PSD so you can make future edits.

I would also add that JPEG is a "lossy" format. This means it compresses the image by removing some data to make the file size smaller. This makes it ideal for photographs and complex images with many colors and subtle gradients. However, because it discards information, the quality degrades with each save. 

Personally, I use PNG because it is a lossless format, which means it compresses the image without losing any data. It preserves image quality perfectly, no matter how many times you open and save it. PNG also fully supports transparency, making it the preferred choice for logos, icons, and web graphics that need to sit on top of different backgrounds.