Hey @SandyPanda , it's possible that the Apex Class you're trying to call is not accessible outside of the managed package. Apex Classes in managed packages are typically protected and cannot be freely called outside of that package.
I found this helpful Salesforce developer forum discussion: the class and methods need to be set as Global in the managed package. If these are not Global, then you will not be able to access them outside of the managed package.
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