Ok. I do not know what went wrong with your addition, but I have created a profile for him.
So now you should go to your father's profile, go to edit, and then on the right: add father. Then do NOT enter data, but enter Thomas-45875 in the most upper entry box.
Then the next step is to create your grandmother and add their proven marriage date. Please make sure there is not already a profile for her.
Please edit the birth location and add sources. Also for your father: please add town, state, country etc (I am guessing you are form the USA, but that is a guess).
Let me know if you need anything else.
(edit) PS: if you tried to add your grandfather to your profile as your father, then that was the cause of the error. You have to go into your father's profile and from there add him.