After editing the script or voice, please make sure to click the PLAY button to apply the changes (once the line turn green, it means the changes have been applied) and check everything before you hit Render.
The character limit for the script depends on your subscription plan. On the free plan, it is 1,000 characters, including spaces; on the Starter plan, 2,000 characters, including spaces; and on the Pro and AdMax plan, 8,000 characters, including spaces.