TL;DR The initialize
instruction is not supported by IBM Quantum Circuit Composer. However, you can use the transpiler to reconvert it in terms of the supported basis:
circuit = transpile(circuit, basis_gates=["u3","u2","u1","cx","id","u0","u","p","x","y","z","h","s","sdg","t","tdg","rx","ry","rz","sx","sxdg","cz","cy","swap","ch","ccx","cswap","crx","cry","crz","cu1","cp","cu3","csx","cu","rxx","rzz","rccx","rc3x","c3x","c3sqrtx","c4x"])
The long version:
The IBM Quantum Circuit Composer does not allow gates outside of the "standard library". The IBM Quantum Lab raises an error explaining the situation:
from qiskit import QuantumCircuit
from ibm_quantum_widgets import CircuitComposer
circuit = QuantumCircuit(1)
circuit.initialize([0, 1], 0)
editor = CircuitComposer(circuit=circuit)
editor

You can use transpile
to rewrite the circuit in terms of the supported gates in the standard library. Here is a generic way to do that:
from qiskit import QuantumCircuit, transpile
from ibm_quantum_widgets import CircuitComposer
circuit = QuantumCircuit(1)
circuit.initialize([0, 1], 0)
circuit = transpile(circuit, basis_gates=["u3","u2","u1","cx","id","u0","u","p","x","y","z","h","s","sdg","t","tdg","rx","ry","rz","sx","sxdg","cz","cy","swap","ch","ccx","cswap","crx","cry","crz","cu1","cp","cu3","csx","cu","rxx","rzz","rccx","rc3x","c3x","c3sqrtx","c4x"])
editor = CircuitComposer(circuit=circuit)
editor

This transpiled circuit is dumplable as QASM too:
print(circuit.qasm())
OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];
ry(pi) q[0];