FFmpeg removed libfaac support in 2016 because the other AAC encoders are better. Use a different encoder.

Native FFmpeg AAC Encoder (-c:a aac)

Use the native, built-in FFmpeg AAC encoder:

ffmpeg -i input.wav -c:a aac output.m4a

• Included by default in all ffmpeg versions.
• Has many features, fairly good quality, supports the most channel layouts and sample rates.
• See more details and specific options with ffmpeg -h encoder=aac.

Fraunhofer FDK AAC Encoder (-c:a libfdk_aac)

ffmpeg -i input.wav -c:a libfdk_aac output.m4a

• Provides excellent quality.
• Supports HE-AAC.
• Not compatible with the GPL, and therefore not often made available by distributors, so you’ll need to compile ffmpeg to use it.
• See more details and specific options with ffmpeg -h encoder=libfdk_aac.

Apple AudioToolbox AAC (-c:a aac_at)

• Only available for macOS/iOS users.
• See more details and specific options with ffmpeg -h encoder=aac_at.

Microsoft Media Foundation AAC (-c:a aac_mf)

• Only available for Windows users with ffmpeg compiled with --enable-mediafoundation.
• See more details and specific options with ffmpeg -h encoder=aac_mf.

Also see:

• FFmpeg Wiki: AAC