You need to send the Exit Safe Start command (0x83), which is what clicking the Resume button in the Simple Motor Control Center does, before setting the speed in your code. The Exit Safe Start command is required before the motor can run if the Input Mode is Serial/USB and the Safe Start mode is not disabled. For more information about that command, see its description under the “Binary Commands” section of the SMC user’s guide.
If sending that command doesn’t help, can you send the settings file from your SMC and pictures showing how you have everything connected in your setup (e.g. limit switches, power supply, and motor)? To save the settings file, click File->Save Settings File… inside the Simple Motor Control Center.