Emily Dickinson's poem "Because I could not stop for Death--" certainly does contain a definite rhyme scheme; however, it's a very nontraditional and very complicated rhyme scheme.
Rhyme scheme is typically identified by looking at the final words of lines in stanzas and assigning letters to each word, specifically matching letters to all words that rhyme. There are more non-rhyming words than rhyming, but all except the third stanza contain one rhyme. If we were to assign letters to each line of each stanza, the letters would go as follows: ABCB DEFE GHIJ KLML NOPO QERE
To break that down even further, the exact rhyme scheme is as follows: BB EE LL OO EE.
To define the rhyme scheme even further, the scheme contains a complex mixture of different kinds of rhymes: end rhyme, eye rhyme, half rhyme, and identical rhyme.
End rhyme is the most common type of rhyme to appear in poetry. It's formed when we rhyme the last syllables in a rhyme in sound, even if the words are not spelled similarly. The editors of Poetry magazine give us the following example from Virginia Hamilton Adair:
And here on this turning of the stair
Between passion and doubt,
I pause and say a double prayer, ... ("Rhyme")
End rhyme appears in Dickinson's first stanza of the poem, when she rhymes "me" with "Immortality" but nowhere else.
In the second stanza, we have her first usage of eye rhyme, which occurs when we rhyme words in spelling only, not in sound. Arizona State University gives us the example of rhyming why with envy ("Glossary of Rhymes"). Similarly, in Dickinson's second stanza, both "away" and "civility" end with the same spelling; she repeats the same pattern in the final stanza with the words "day" and "eternity."
Half rhyme occurs when only ending consonant sounds are rhymed. The editors of Poetry magazine give us the example of rhyming "'tell' with 'toll,' or 'sopped' with 'leapt'" ("Rhyme"). In her fourth stanza, Dickinson rhymes "Chill" with "Tulle," which both have double L consonant sounds.
Her last rhyme, other than her repeated use of eye rhyme in her final stanza, is identical rhyme, which happens when identical words are rhymed through using the exact same word twice. In the fifth stanza, Dickinson creates rhyme by repeating the word "Ground" twice.