The disability to walk in a straight line is a symptom of what condition

It would be called a vestibular disorder or disability

Ataxia.

Ataxia is the loss of full control over your bodily movements. There are different kinds of ataxia.
People describe it as the lack of muscle coordination when a voluntary move is tried..
Symptoms:
Difficulty walking, you stagger.
Speech = slurred & slaw
Difficulty swallowing
Examples:
Difficulty using the hands and fingers to button shirts, or pick up a fork or spoon to eat. Writing, keyboarding, sewing, and playing an instrument can also be difficult.