If you face this error that means that you are running Swift version inferior to 5.5 and Xcode 12 or lower.
The 20th of September 2021, Apple has introduced Xcode 13 with Swift 5.5.
Every binary built with Swift 5.5 is not compatible with earlier version, meaning everything not built with Xcode 12 and lower.
Purchasely SDK for Xcode 13 and Swift 5.5 was released on the first day Xcode 13 was released first on a specific branch and on the master branch by October 14th as we saw that most developers migrated to Xcode 13.
Two things are possible to solve that issue:
Update to Xcode 13 (recommended)
Force Purchasely SDK to stick to version to 2.7.2. You won't benefit from new features and fixes so this should only be a temporary solution for you.