I am right now a comdominium unit owner and the condominium complex is planning to make major repairs and is now going to charge unit owners for an assessment for the repairs. Can I use this assessment as a deductible expense?

No - not unless the condo is part of your business, which seems unlikely. Otherwise about all you can do is add the cost of major repairs and capital improvements to your cost basis for the condo, so that later when you sell it if you have a taxable gain on the property you can reduce the amount of capital gains tax owed.