Is there a way to do that? Moveobjects on doesnt affect placing Fences... I want to put an invisible fence onto a low fence cause I dont want Sims to walk over the low fence :/

No, not currently.

Would probably be easier to remake the low fence into a regular fence, or possibly change some settings for the game to recognize it as a regular fence. I don't make fences, so maybe someone else knows how?

I feel like there are omsp blocks that prevent sims from walking on tiles by silentlucidity?

Otherwise to make a fence walkable you edit the XFNC. Look for the line maywalkover line and change the value from true (0x00000001) to false (0x00000000).

Do I copy the fence to do that with SimPE?

If it's a maxis fence, just clone it and create a new one. If it's a cc fence you can just edit it in simpe.

Okay thank you very much!

If it's for sth simple, I took an idea from Plasticbox's house https://modthesims.info/d/532413/swinton-5br-3ba.html (see third image where it shows the rear porch). On sims 2 I was able to make it look like the stone wall on the ground with a metal railing over it. Need to place a floor tile to draw the "higher" fence, delete the tile and draw the "lower" fence.

Depending on the fence and the area you can place a fence on top of a fence. I did so in this lot: https://modthesims.info/d/499887/ma...or-cc-free.html I haven't done so in years but I used the foundation tool. That won't work with low fences though.