No, there is not a particular rule against being able to claim a loss on a rental property that was converted from personal use to rental use, as long as other requirements are met to be able to claim a loss.
Take a look at the information that you entered with regard to personal use days. The entry should be zero personal use days unless you used the property AFTER the date it was converted to a rental property. Any personal use up to the date it was converted does not count. This information can be found as you go through the Property Profile section for the rental property.
If this is not the issue, come back with a follow-up question and someone will try to help further.
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