A person who cannot understand that there may be other perspectives

Solution 1:

I think narrow-minded could come close to what you are looking for.

Lacking tolerance, breadth of view, or sympathy; petty.

Solution 2:

You could say that they're blinkered:

Having or showing a limited outlook

You could also say that such people exhibit tunnel vision.

Solution 3:

Close-minded or closed-minded. Various equivalents in various situations might include bigoted or self-righteous. Likewise, fanatical.

Solution 4:

Also see terms mentioned in response to the question Single word for people who are like "a frog in the well"; eg:

Synonyms of parochial, "characterized by an unsophisticated focus on local concerns to the exclusion of wider contexts", may serve. Such synonyms include confined, insular, limited, local, narrow, narrow-minded, petty, provincial, regional, small-minded, isolated.

Insular, for example, has a sense that means "separate or isolated from the surroundings; having little interaction with external parties; provincial." Narrow-minded and small-minded are of course easily understood, and so would be close-minded.

Blinkered, fitted with blinkers, or "eye shields attached to a hood for horses, to prevent them from seeing backwards and partially sideways", is another possibility, as are formations from synonyms blinders and winkers.

... monomania, "excessive interest or concentration on a singular object or subject" or "a pathological obsession with one person, thing or idea"; obsession, "compulsive or irrational preoccupation; an unhealthy fixation"; idée fixe; tunnel vision, "an extremely narrow point of view; narrow-mindedness"; and locked-in.

in one answer, and the following terms in other answers:

• myopic, “2. shortsighted; improvident; 3. narrow minded”
• solipsist, “One who adheres to self-absorption and an ignorance of the views or needs of others”
• short-sighted, “(figuratively) Unable to see long-term objectives; lacking foresight.”

Solution 5:

hidebound

hidebound: not willing to accept new or different ideas

unreconstructed

unreconstructed: used to describe someone who has strongly held opinions and beliefs that have not changed even though they have been criticized or have become unpopular

sectarian