GPS Invalid Message & Also a No ATC Problem

Hi   have a problem  using my 737NGX   tried to fly the Tutorial 1  EGKK to EHAM  but each time I get the L GPS or R GPS Invalid message on FMC and audible alarm , yesterday flew RJGG to VHHX   and  LFPG to EGLL with no such problems,  have programmed all data as previous please can anyone advce what i'm failing to do? I am using default airports not Add-Ons for this Tutorial.

Could I also ask i've just recently installed my FSX Gold Edition & Accelerator & PMDG 737NGX and I have no ATC voices I do not have FS2 crew just the usual ATC with the FSX  again I have set up for pilot voice opposed to text in FSX and all sound sliders are set as advised in the PMDG manual  again can I seek some advice please 

My specs:

Windows 7 Ultimate 63Bit

CPU Intel i5 4670

16GB

Graphics NVIDIA GeForce  GTX 760 2GB

Hard Disk SSD 120GBs

                                                 Many Thanks

                                                  Richard