The Arm A64 Instruction Set Architecture manual says that the MOV (element) instruction takes the form MOV <Vd>.<Ts>[<index1>], <Vn>.<Ts>[<index2>], where <Ts> is one of B, H, S, or D. Only certain other instructions accept a number in front. GNU as allows you to include it for any instruction, but this is non-standard. This is explained at https://stackoverflow.com/questions/71907156.
The Arm A64 Instruction Set Architecture manual says that the MOV (element) instruction takes the form
MOV <Vd>.<Ts>[<index1>], <Vn>.<Ts>[<index2>], where<Ts>is one of B, H, S, or D. Only certain other instructions accept a number in front. GNU as allows you to include it for any instruction, but this is non-standard. This is explained at https://stackoverflow.com/questions/71907156.Disclaimer: My assembly knowledge is close to 0.