Root composer.json requires mocking-magician/coinbase-pro-sdk ^0.9.4 -> satisfiable by mocking-magician/coinbase-pro-sdk[v0.9.4].
mocking-magician/coinbase-pro-sdk v0.9.4 requires symfony/yaml ^4.0 -> found symfony/yaml[v4.0.0, ..., v4.4.34] but it conflicts with your root composer.json require (5.3.*).
It does not look like the package supports the latest version of Symfony. I haven't tested updating the composer.json file.