Can I claim a terminal loss if I sell one rental property but still own others in the same CCA class?

Not right away. A terminal loss only becomes available once you dispose of the last property remaining in a particular CCA class. If you sell one rental property below its share of the class's UCC but still own other properties pooled in that same class, the loss isn't triggered - the UCC pool simply continues on with the remaining properties, and the shortfall is effectively absorbed into the ongoing balance rather than being deducted immediately.

This surprises people who expect a loss the moment a specific property sells for less than they think it's worth on paper, but CCA classes work on a pooled basis rather than tracking each property separately by default. The loss you were expecting doesn't disappear entirely - it's deferred, and will only crystallize as an actual terminal loss once the very last property in that class is eventually sold or disposed of, at which point the full remaining UCC shortfall becomes deductible against any income. Because this can mean waiting years for a loss you were counting on sooner, some property owners choose to elect to keep a rental building in its own separate CCA class specifically to avoid this pooling effect, which is addressed in a related question.