[HELP] how to encode variable bitrate AAC audio?

[Selur]

a MP4's audio is E-AC3, when enable cut support, then encode, Hybrid displayed error

Not sharing the error message -> nothing I can say about it. Mp4 itself nowadays does support e-ac3 muxing, but there might be a but with the detection on passthrough,...

can i encode with variable bitrate AAC?

Sure,... you just have to either use an encoder which allows to encode to vbr with a given bit rate or which allows to encode using a quality factor.
All the aac encoders (qaac, fhg, faac, NeroAac, fdk, ffmpeg) support one of these options.

Cu Selur