This is a Stata question and is not specific to DHS data, but I will offer a brief response.

It is not unusual to get a message to the effect that some part of svyset cannot be implemented. I suggest that you repeat svyset with fewer components (in this case, without the pweight component), and repeat the model, until you get a result, in this case the brant test. You then insert a statement in your write-up saying that you had to do this.

An argument can be made that tests should be unweighted anyway. Also, DHS surveys do not usually have enormous variation in the weights, and you will usually reach the same conclusion about significance or non-significance whether or not you use the weights.

I won't be surprised if this specific limitation is removed in some future version of Stata.