Photoshop insists that file must be saved as PSB

​@SamSegar 

When you save as a PSB, what is the file size in Windows File Explorer?

Jane

At those dimensions, six layers and 16-bit colour, I wouldn't expect the file to require PSB unless there's something else increasing its size.

Adobe's documented limits are 2 GB for PSD and 4 GB for standard TIFF. So the dimensions alone certainly don't explain the warning. https://helpx.adobe.com/photoshop/desktop/save-and-export/save-files/save-large-documents.html?

What’s the resolution? Could be pushing it up the ladder to the file format it can support.
 

A few things I'd check

Edit > Customize Save Dialog File Formats

https://helpx.adobe.com/photoshop/desktop/save-and-export/save-files/customize-the-file-format-list.html
This feature was introduced in Photoshop 27.10 and allows formats to be hidden.
Make sure TIFF is enabled, or click Restore Defaults.

Try File > Save a Copy rather than Save As.

At the bottom of the document window, change the status bar display to Document Sizes. What are the two values shown? The second includes the layers and channels and should give us a better indication of the document's actual size.

Check for embedded Smart Objects, additional alpha channels or unusually large layer contents extending beyond the canvas.

Try Image > Duplicate, flatten the duplicate and see whether TIFF becomes available. Don't flatten your original.

If flattening resolves it, we can investigate which layer is responsible.

I'd also be interested to know whether you can create a new 3461 × 3461 pixel, 16-bit document and save that as TIFF. That would help determine whether the problem is document-specific or a Photoshop setting.

What are the two figures shown under Document Sizes?