Issue
When you export a file using Adobe Photoshop's GIF89a Export
plug-in, the transparency options are unavailable in the GIF89a
Export dialog box.
Solution
Convert the file to Indexed Color mode before choosing File >
Export > GIF89a Export.
Additional Information
Photoshop's GIF89a Export plug-in enables you to export and save
RGB or Indexed Color files in GIF89a format. When you export an
Indexed Color file, you can select one or more colors as
transparent, designate an additional channel to use as a
transparency mask, and select a transparency index color in the
GIF89a Export dialog box. When you export an RGB file, these
transparency options are unavailable.
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