The error occurred because the fiscal year-end should be December 31 the year you sold the property.
The "from" area for the fiscal period is the date that your operation began. (If your operation began before January 1st, then input January 1st of the current year.)
The date of disposition on the Capital Gains form should be the date that you sold the property.
Information on Fiscal Period for Form T776, as per Canada Revenue Agency (CRA):
Completing Form T776, Statement of Real Estate Rentals - Canada.ca
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