There is nothing grammatically wrong.
Some style guides suggest that the active voice is preferred over the passive, when both are possible (example). So "I performed an experiment" is stylistically better than "An experiment was performed"
This guide is probably a response to the overuse of the passive voice in some technical literature when the author is trying to sound objective, but ends up sounding dull. Allen Downey (an author of computer science books) has written a short essay about this.
The grammar checker in Word in particular declared war on the passive voice, flagging it as if it were a grammatical error, which it is not. The grammar checker from earlier versions of Word was known to be fragile. It allowed some incorrect grammar to pass, while flagging stylistic things like the passive voice. I understand that it has improved.
It is possible to configure the grammar checker to allow the passive voice, or you can ignore the warning.