Can anyone explain why this is a blunder? I am creating a forward anchor and hindering my opponents plans to make 6 point prime

Maybe because after your play it would be easier for your opponent to play behind you?

I assume you want maximum contact since you're so behind in the race and an advanced anchor like that isn't ideal?

Over the board, I would have made the anchor like you.

What uberkuber said, plus you need to not only hit a shot but contain it as well. Your board still needs a lot of work. The 13/10 part will bring a builder into play to help with that.

But yeah, OTB I’m pretty sure I’d make the same blunder and anchor on the five.

I assume this is DMP because the cube hasn’t been turned (and it would be D/P at 0-0 in any other match length). This means your gammon losses don’t hurt you so you can afford to stay on the 23 pt to maximize your wins at the expense of (meaningless) gammon loses. This is probably why it’s so big. If you owned the cube at 0-0 in a (say) 7 pt match I am guessing the plays would be a lot closer.

Also, the point of making the advanced anchor isn’t to stop your opponent from making a prime — you’re not interested in breaking your anchor any time soon, you are trying to sit back and wait for a shot since you’re down 65 pips. Your only plausible path to winning this game is to build a strong prime (or close your board) then hit a shot while blue is trying to bear in, and staying on the 23 pt produces the most shots and must win the most games. The only reason I would consider making the 20 pt at certain scores is to cut down on gammon losses, or if the race were much closer (say within 20 pips) where at least sometimes we can win by racing.