Hi Madeleine!
Since we don't have your data, it is hard to reproduce your problem, so please have a look at this thread to see how you can provide us with a dataset to reproduce your error.
Without knowing your data, it seems like your acc vector contains constant values, i.e. always the same value. In glmer, the argument left of the tilde (~) is the response argument. So maybe check the vector acc to see wether it only contains one value (either 0 or 1 I suppose).