Dear @Han.Yiqi,
So, did setting PtfmYIner nonzero solve the numerical instability you are seeing?
When you enable both PtfmYDOF and YawDOF in ElastoDyn, it is necessary to set PtfrmYIner nonzero as discussed recently in the following forum topic: Wind Veer Parameter - #55 by Jason.Jonkman.
Best regards,