What is wrong with this sentence? Since I no longer maintain an office downtown for working on this kind of project.
I say fragment
It's a fragment -- not a complete sentence. It's a dependent (subordinate) clause. It needs an independent clause attached to it.
http://grammar.ccc.commnet.edu/grammar/fragments.htm
The main issue with the sentence is a lack of clarity and coherence. It seems to be a fragment or a sentence that has been cut off. Additionally, there is a problem with verb tense consistency. Here's a possible revision:
"Since I no longer maintain an office downtown for working on this kind of project, it has become challenging to find an alternative location."