It's not cut and dried, but a good guideline is this:
If you've written a program, but it doesn't work yet, and you've realized you made major architectural mistakes or whatever, feel free to rewrite from scratch.
If you've written a program and it's been in real use for a while, solving real problems for a lot of happy users, that means it embodies a lot of knowledge that will be difficult to re-create from scratch; in that case, think very carefully before embarking on a rewrite, and look hard for a way forward that will preserve that knowledge.
If you've written a program, but it doesn't work yet, and you've realized you made major architectural mistakes or whatever, feel free to rewrite from scratch.
If you've written a program and it's been in real use for a while, solving real problems for a lot of happy users, that means it embodies a lot of knowledge that will be difficult to re-create from scratch; in that case, think very carefully before embarking on a rewrite, and look hard for a way forward that will preserve that knowledge.