In A1111 settings, set "File format for images" to jpg, then run the extension.
The issue:
By default, it is trying to save the image in RBGA format (with an alpha channel.) JPG doesn't have an alpha channel, so there is an error. I don't think this alpha channel is needed, so it may be beneficial to remove it before saving to reduce storage space.
To reproduce:
In A1111 settings, set "File format for images" to jpg, then run the extension.
The issue:
By default, it is trying to save the image in RBGA format (with an alpha channel.) JPG doesn't have an alpha channel, so there is an error. I don't think this alpha channel is needed, so it may be beneficial to remove it before saving to reduce storage space.