You are attempting to merge cases from a list view using the Case Merge Premium "Merge" button, and encounter the following error: Error merging cases from list view: core.apexpages.exceptions.ApexPagesHandledException: Object type not accessible. Please check permissions and make sure the object is not in development mode.

This error is due to the following Salesforce known issue, which is unfortunately marked with a status of No Fix.

That said, this error can be avoided by making a couple of changes to your Case List View where the error was originally encountered.

1. On the Edit page of your Case List View under "Step 2. Specify Filter Criteria", remove any instances of "NOT" from your Filter Logic.

2. Under "Step 3. Select Fields to Display", ensure the top field in the Selected Fields column is not of the Multi-Select Picklist, Rich Text Area, Long Text Area or Encrypted field type.


If adjusting your Case List View configuration is not a viable option, consider recreating the "Merge" button as a JavaScript button (depicted below).



JavaScript for easy copy/paste:
 

var records = {!GETRECORDIDS($ObjectType.Case)};
var url;

if (records.length < 2){
alert("You must select at least two cases to merge.");
} else if (records.length > 9){
alert("You have selected too many cases to merge.");
} else {
url = '/apex/csmrgp__csmrgselect?';

for (var i = 0; i < records.length; i++){ 
url = url + "cs" + (i+1) + "=" + records[i]+ "&";
} 

if (typeof srcUp == 'function') { 
srcUp(url + 'isdtp=vw');
} else {
window.location.href=url;
}
}