Why can't you try for baby?

Hello
Check that the pregnancy settings for your sims allow that one can become pregnant and the other one can make others pregnant.

If you have cc/mods in your game, move your mods folder to the desktop. Then start the game and test to see if the issue is still there. If it has helped with the issue, put your mods/ CC back into the newly generated mods folder that is in the sims 4 folder one by one or in batches and testing after each one to see which is the causes of the problem.

The sims need to be aged young adults or older.

Well thank-you again @rosemow, I thought I had checked the settings b4 I put sim in game. That's what I get for just downloading to game a sim I didn't create.

You're welcome :)

How do I go the the pregnancy settings?

Pregnancy settings are available in CAS. There should be 3 dots (...) you can click on under gender options where you can choose if the sim can get pregnant, get other sims pregnant or neither.

Please don't necropost in old threads. I'm going to go ahead and close off this thread now as it has been inactive since 2016, If you have any questions about necroposting you can read this thread here : Necroposting