Welcome to Bass Buzz @dualspiral66
Slapping these strings can be very frustrating because of muting. it feels impossible to slap these strings to get any sound, without making all sorts of racket on the E and A string.
Two muting techniques might help, along with re-positioning of your right hand when you are slapping.
1 - palm mute the E string with your right hand, sort of reset your hand on that string while you are slapping. if you can get the A string with the heel of your palm as well, thats a plus, but you should at least be able to get one string.
2 - place your index finger flat across all 4 strings, and use your middle finger, or ring finger, or pinky, which ever feels most comfortable, to fret the note. your finger is behind the fretted note, so it mutes all the other strings.
once you get this down, you won’t feel so reluctant to slap the D and G string.
also, if you move your right hand towards the bridge, away from the neck a bit, so you are slapping in about the middle, or even closer to the bridge. This is where I get the most volume out of a slap.
I am no expert, but this is what works for me, and I get a little crazy watching some of the experts barely touch a string and get crazy sound out of it. But that will come in time, start with this and see if it helps.