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."

The sentence is missing a main clause, which is necessary for a complete sentence. It begins with a subordinate clause ("Since I no longer maintain an office downtown for working on this kind of project"), which cannot stand alone as a sentence because it does not express a complete thought. To improve the sentence, it needs to be revised by adding a main clause that provides the necessary context and completes the thought.