I believe you hit the nail on the head - not enough roots. Curious as to what it's growing in. Cactus definitely do best with 1/2 inorganic material like gravel and or perlite. The pot looks rather large for the size of the plant. I would only sprinkle water on the surface (not deep watering) near the cactus roots til it starts to firmly grasp the mix on its own. Maybe a small stick like a popsicle stick, coffee stirrer or such can be used to prop the cactus up until it forms it's own rootlets.

A few little sticks can keep it up until it grows new roots. I often use bamboo skewers even though mold has a tendency to grow on them. Your cactus looks small enough you probably can use toothpicks.

Likely not enough roots so there is nothing to help it 'stand up'; too much water should be avoided (if no roots, plant can't take any water in) and def. change the soil - lots of gravel and perlite and little soil. Add more soil so it is not so deep inside of pot. You could also use few larger rocks to support it while growing roots.

I use tennis ball-sized rocks to support jade branch cuttings until roots are established. The less the plant is jarred/disturbed, the faster the roots anchor. The soil getting dry stimulates root growth, so let the soil dry well between waterings. I echo others' advice to reduce the ratio of peat through adding grit or perlite.

ez

Thank you! :)