You will be able to find a range of free and paid for online training via Google. You can find some recommended resources below.
Free YouTube videos
The INTERACT EU project produced two YouTube playlists with 17 short training videos on translation and post-editing for volunteer translators, see here: https://www.youtube.com/channel/UCRbg0k_1W8KU1xDFqZjDZtg/featured
The videos are designed specifically for volunteer translators in humanitarian crises contexts, but they are a good introduction for any volunteer translators.
TAUS e-learning
TAUS is an independent organization which shares knowledge, metrics and data to support people and businesses doing translations and related activities. TAUS offers a range of resources and training, including online courses on post-editing and quality management of translations: https://elearning.taus.net/course/index.php. These courses come with a fee.