It all depends on your definition of Aikido
Yes one can and should train by themselves. This may involve something as basic as practicing centering throughout the day. Quite frankly, I tell my students to practice "shopping cart Aikido". This involves using the basic fune kogi movement in moving a shopping cart around a store. Instead of pushing from one's shoulders, move from your hips.
When my grandchildren come to visit and play "jump on grandpa", I use quite a bit of tenkan to keep them from hurting me and themselves. Their interpretation is that they are getting swung through the air.
Really training is where you find it, not confined to a "dojo". The world should be your dojo. Your center moves with you. Keep your awareness with you also. This is the fundamental Aikido training. Throws, pins and take downs are other things we do also.