Here is example code that produces the given error message:

{% include_relative ERROR_ModifiesValue.dfy %}

The expression in the modifies clause is expected to be a set of object references. It is permitted also to be a comma-separated list of object references or sets or sequences of such references. In this example, the expression is none of these; instead it is a map. maps are values and so are not modified; new values are created, just like an integer is not modified --- one computes a different integer value.

If the intent here is to say that any of the A objects stored in the map may be modified, then one has to construct a set of all those objects. For a map, there is an easy way to do this: just say m.Values, as in this rewrite of the example:

{% include_relative ERROR_ModifiesValue1.dfy %}