We only allow exporting to TGA if the texture source is of the correct format. It must be BGRA8 or RGBA16
Export for certain marketplace asset limited to .COPY and .T3D (and .BMP in 4.24)
I was unable to reproduce this with a texture that I created even with Grayscale / sRGB disabled.
Place attached .uasset (texture) in content folder.
Right-Click on texture and attempt to export.
Result: 4.23 - You can only export as .COPY and .T3D 4.24 - You can export as .BMP in addition.
Expected: Able to export as .TGA
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-85797 in the post.