CF'r went down for 9 days the 1st game of the play-offs. I put  him on the 15 day DL thinking I would be able to replace him. When I try to make a roster move I get: "Error: None of your roster moves could be added due to the following reasons: Playoff rosters can only be changed prior to the start of the next round of the playoffs."
So do I have to finish this series with a 24 man roster or am I missing something?

That is correct.  You can only make roster changes in between playoff series.

True, I had a LRA that went down earlier in the season and he came off the DL during a playoff series but couldn't be activated and available until after I won the series and moved on.