Then , I believe there is error in your XML . It should be as below . Also its recommended not to use 'nms' namespace for creating any custom schema. If you still want with nms namespace, try deleting your schema and recreate as shown below . else try creating with different namespace as per your need and this should resolve your issue.
<srcSchema _cs="Recipients index (nms)" created="2018-06-19 10:06:12.190Z" createdBy-id="1133699433"
desc="Recipients index" entitySchema="xtk:srcSchema" img="" label="Recipients index"
labelSingular="Recipients index" lastModified="2018-07-13 07:38:49.082Z"
md5="2B5B0D3667159163610E2DAC2AB51559" modifiedBy-id="1133699433" name="recipientsIndex"
namespace="nms" xtkschema="xtk:srcSchema"/>
<element label="Recipients Index" name="recipientsIndex"/>
</srcSchema>
Thanks,
Prem Kumar Ganesan