My accountant last year depreciated an asset over 15 years at 200db. I guess you can't do that. Would it be an issue to file 15 years at 150db this year?

No, that is exactly what you need to do.  Because you had excess depreciation in the past be sure to correct the combined total of all depreciation you used in the past when that screen comes up.  Keep your records so that when and if you dispose of this asset you have the correct depreciation expense used on your tax returns.

Thank you!!!