I can't run depreciation for my last period, how can I fix this? 

When you perform the depreciation run for the last period, you cannot use the regular "Depreciation Run" function.  There is a special function for running the Year End.  You can perform the Year End Run by going to Activities>Year End Run.  You will also find the Year End Update function located there as well.

Once you perform the Year End Update, you can now use the Depreciation Run function for your next period.