AVIF is an image file format that store both still and animated images. The animated AVIF use AVIFs as the file extension. It uses lossless or lossy compression. AVIF stands for AV1 Image File Format on account of its use of AV1 compression.

Make sure you have selected the animated GIF or WebP files instead of still images. After that, you can choose the AVIF as the output file format to convert GIF/WebP to AVIFs.