The error means that somewhere in a Route, you specified something like
[Route("SomeRoute/{someparameter:string}")]
“string” is not needed as it is the assumed type if nothing else is specified.
As the error indicates, the DefaultInlineConstraintResolver that Web API ships with does not have an inline constraint called string. The default supported ones are the following:
// Type-specific constraints
{ "bool", typeof(BoolRouteConstraint) },
{ "datetime", typeof(DateTimeRouteConstraint) },
{ "decimal", typeof(DecimalRouteConstraint) },
{ "double", typeof(DoubleRouteConstraint) },
{ "float", typeof(FloatRouteConstraint) },
{ "guid", typeof(GuidRouteConstraint) },
{ "int", typeof(IntRouteConstraint) },
{ "long", typeof(LongRouteConstraint) },
// Length constraints
{ "minlength", typeof(MinLengthRouteConstraint) },
{ "maxlength", typeof(MaxLengthRouteConstraint) },
{ "length", typeof(LengthRouteConstraint) },
// Min/Max value constraints
{ "min", typeof(MinRouteConstraint) },
{ "max", typeof(MaxRouteConstraint) },
{ "range", typeof(RangeRouteConstraint) },
// Regex-based constraints
{ "alpha", typeof(AlphaRouteConstraint) },
{ "regex", typeof(RegexRouteConstraint) }