I am invoking an approval process in an after update trigger which results in bunch of runtime errors. The approval process works flawlessly when submitted through default "Submit for Approval" button

below is my trigger code and the run time error messages 

trigger Approval_Process on SPA__c (after update){

for(SPA__c sp: Trigger.new){

if (sp.Submit_for_Approval__c){

Approval.ProcessSubmitRequest req1 = new Approval.ProcessSubmitRequest();

req1.setComments('Submitting request for approval.');

req1.setObjectId(sp.id);

req1.setNextApproverIds(new Id[] {UserInfo.getUserId()});

//req1.setNextApproverIds='005Q0000000IFNf';

Approval.ProcessResult result = Approval.process(req1);

System.assert(result.isSuccess());

System.assertEquals('Pending', result.getInstanceStatus(), 'Instance Status'+result.getInstanceStatus());

}

}

}

 The runtime errors I get are

- CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY

- NO_APPLICABLE_PROCESS

The detailed message is below

Error: Invalid Data. Review all error messages below to correct your data.Apex trigger Approval_Process caused an unexpected exception, contact your administrator:

Approval_Process: execution of AfterUpdate caused by: System.DmlException: Process failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY,

Approval_Process: execution of AfterUpdate caused by: System.DmlException: Process failed. First exception on row 0; first error: NO_APPLICABLE_PROCESS, No applicable approval process found.: [] Trigger.Approval_Process: line 11, column 45: []: Trigger.Approval_Process: line 11, column 45