But they're omnipotent, shouldn't they still be able to make it?
Infinity is not just a number, it is a concept.
Technically, infinity is equal to infinity, and can't be changed, and doesn't comply to normal mathematical operations. So you can't say
x=max weight god can lift=infinity
y=x+1
Because infinity is equal to infinity plus one.
Also, a god probably would not actually physically lift an object, rather, manipulate the atoms as to raise the object.