Hi Xianyao,
I suggest that you write your code instead of uploading a picture of it, possibly a reduced version of it, just to reproduce the error.
The problem might be that the value n is probably a symbolic value used by RecurrenceTable, you can't assume that it is an integer or something you can use for indexing unless it's clearly stated the data structure being used.
Also, not related to your problem, the initial values should a[1],b[1].
Cheers!
