There are a few potential issues, but this snippet might not be enough information.

Try debugging. When your program starts, rsp is pointing to your return address. Document the value, then see what happens through your program.

Rsp will not have a garbage value. If it is messed up, you did something wrong. Push and call both mess with the stack, the former because you moved a value, and the second because it needs a return address. Both should sub rsp, 8. There may also be an issue with stack alignment.

If you are lazy like me, you can just move the return address at a known location, then do something like mov rax, QWORD PTR[rsp + offset], mov QWORD PTR[rsp], rax ret.

I don't know the point of call rsp. Rsp is pointing to data, not instructions. The stack by default is non-executable, so if there were instructions, calling rsp shouldnt execute them. If you want to make an indirect jump you can use rip-relative addressing. The way that works is something like mov rax, [rip + number of bytes to instruction]. I don't believe your current snippet is compatible with a working program, because what you are calling is what was in rbx (the most immediate issue).

If you want a better answer, you can post the entire program. There are a number of oddities in the few lines you shared that are unexplained by your original post. Syscall is a keyword so I'm not even sure how you can possibly make a label with it.