The last but not least reason
Solution 1:
No. Last but not least is an idiom:
last but not least idiom
—used to say that a final statement is not less important than previous statements
// The television is big, has an excellent picture, and last but not least, it’s cheap.
It functions adverbially, and it’s used like this:
The television is big, has an excellent picture, and last but not least, it’s cheap.
Last but not least, I am applying for this position for the opportunity to grow in the consulting field.
You can swap in another adverb to see:
The television is big, has an excellent picture, and finally, it’s cheap.
Finally, I am applying for this position for the opportunity to grow in the consulting field.
You can also look at this another way: Just as you wouldn’t say the most reason, neither would you say the least reason.
Solution 2:
Cambridge
Last:
(the person or thing) after everyone or everything else
the least expected or wanted person or thing
Hence, The last reason is, by implication, the final or least important reason in a list that you have given, defined or implied.
You may reasonably qualify one adjective by another. In this case you chose to qualify last by not least, making clear that, although last in position, it is not least in importance.
The use of but refutes any supposition in the mind of the reader that the last is the least important.
Your resultant noun phrase last but not least reason may be positioned anywhere that nouns are positioned: beginning, end, object, subject and other roles in sentences.
Solution 3:
Avoid using "last but not least + noun” e.g "last but not least motivation”, instead use an adjective.
Last but not least exciting, is the opportunity to grow in a consulting company.
From Google Books
Last but not least exciting are the athletic and battle-ax competitions, which originated as martial exercises under King Malcolm Canmore in Scotland around 1060.