Access denied while deleting an entity
If you look at EntityAccessCheck::access() you'll see this:

$requirement = $route-&gt;getRequirement('_entity_access');
list($entity_type, $operation) = explode('.', $requirement);


The second line is causing the notice, and your access check to subsequently fail. Basically, the access method is expecting (requiring, in fact) your permission to contain an entity type and access operation, separated by a ..

If you take the node module's routing file as an example:

node.delete_confirm:
  path: '/node/{node}/delete'
  defaults:
    _entity_form: 'node.delete'
    _title: 'Delete'
  requirements:
    _entity_access: 'node.delete'
  options:
    _node_operation_route: TRUE


I would suggest you just need to change your route to match:

mymodule.myentity_delete:
  path: '/admin/config/search/mymodule/myentity/{myentity}/delete'
  defaults:
    _entity_form: 'mymodule_myentity.delete'
  requirements:
    _entity_access: 'mymodule_myentity.delete'