Cannot grant, deny, or revoke permissions to sa, dbo, entity owner, information_schema, sys, or yourself

When you try to give GRANT EXECUTE permission to some SQL Objects then smetimes you might encounter below issue:-

Cannot grant, deny, or revoke permissions to sa, dbo, entity owner, information_schema, sys, or yourself

Resolutions:-
As per the comments, if you're already the db owner of that database, than you don't need to grant any permission for the db.

Now, in order to find out what specific permissions you have, you can use the following queries:

USE AdventureWorks2008R2;
SELECT * FROM fn_my_permissions (NULL, 'DATABASE');
GO