If an expected result is not specified then:
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We cannot run the test
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It may be difficult to repeat the test
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It may be difficult to determine if the test has passed or failed
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We cannot automate the user inputs
Without a specified expected result, it's difficult to determine if a test passed or failed. The test outcome is judged against expectations, so missing expected results makes the test inconclusive.
Without a documented expected result, a tester has no objective basis for deciding whether the actual output matches what was intended, so pass/fail determination becomes difficult or subjective. The test can still technically be run and even automated at the input level — what's missing is the ability to judge the outcome, not the ability to execute the test.