Each tagged release of SwiftPM has to be used with a particular corresponding toolchain. The latest release is compatible with Swift 5.2. When Swift 5.3 was released, SwiftPM never had a version tagged. For more information:
At the bottom of that thread you will find links to a fork that does have a tagged release for Swift 5.3. (The 5.3 and 5.3.2 tags are basically interchangeable and work with any 5.3.x. The more recent one just contains some bug fixes around conditional dependencies.)