Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index

@sujithra

The below code will do the work for you. You can add more fields to the selector following this footstep:

using PX.Data;
using PX.Objects.CR;
using PX.Objects.CS;

namespace PX.Objects.FS
{
    [PXNonInstantiatedExtension]
    public sealed class USRFSServiceOrderExt : PXCacheExtension<PX.Objects.FS.FSServiceOrder>
    {
        public static bool IsActive() => true;

        #region LocationID
        [PXMergeAttributes(Method = MergeMethod.Merge)]
        [PXSelector(typeof(Search2<Location.locationID,
            LeftJoin<Address, On<Address.addressID, Equal<Location.defAddressID>>>,
            Where<Location.bAccountID, Equal<Current<FSServiceOrder.customerID>>,
                And<Location.isActive, Equal<True>,
                    And<MatchWithBranch<Location.cBranchID>>>>>),
            typeof(Location.locationCD),
            typeof(Location.descr),
            typeof(Address.addressLine1),
            typeof(Address.addressLine2),
            typeof(Address.addressLine3),
            SubstituteKey = typeof(Location.locationCD),
            DescriptionField = typeof(Location.descr))]
        public int? LocationID { get; set; }
        #endregion
    }
}

And this is the screenshot from the Location Selector:

If you still face error after this change, you may have issues elsewhere.