Does the file have several layers? With CS5, I think you have to flatten the file to be able to save as jpg. (jpgs can't have layers)
So I suggest that you first make a copy of the file (File > Duplicate), and then flatten the copy.
To post this image online, you have to reduce the pixel dimensions considerably. Since this is a panorama, I suggest that you try something like 2500 pixels on the long side, use Image > Image size to do this.
Now you can use Save for web to create a jpg.
Try a quality setting of 60, and make sure to check Convert to sRGB and Embed profile.
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You need to get the image size down to less than 30,000 pixels wide x 30,000 pixels high before you will be able to save as JPG in Photoshop.
Choose Image > Image Size and leave the check mark on Resample.
Right! Completely forgot about that.
Yup. It's a long standing bug in Photoshop.
That did it thanks.
You're welcome! Glad the problem is solved!