fields =['TR.PriceMoCountryRank(SDate=-30D)'] would return 400 error in
ids, err = ek.get_data(RICS, fields=fields)
As @Zhenya Kovalyov said, there's no history for TR.PriceMoCountryRank field in Eikon. You can verify it using Data Item Browser app or Formula Builder in Eikon Excel: select this field, select Parameters tab and observe there's no Series checkbox at the top of the parameters list. Compare it to the parameters available for say TR.ClosePrice or TR.Revenue where the Series checkbox is shown.
TR.PriceMoCountryRank is a singe value rather then a series, but I suggest double-checking with your Thomson Reuters support desk.