Why this happens
Sequence diagram activation bars are stack-based: activate A (or the shorthand +A on a message) pushes an active lifeline for A, and deactivate A (or -A) pops it. Calling deactivate without a matching activate first -- or deactivating twice -- leaves nothing to pop.
This commonly happens after editing a sequence diagram: an activate line gets deleted or moved, but the matching deactivate is left behind.
How to fix it
Add the missing activate A before the deactivate, or remove the extra deactivate. For simple call/return pairs, the shorthand A->>+B: request ... B-->>-A: response is easier to keep balanced than separate activate/deactivate lines.