Process completed with exit code 1.

The block following this 'let' is unfinished. Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider giving this block an explicit result.

The result of groupByAsync must be consumed with a parallel combinator such as AsyncSeq.mapAsyncParallel. Sequential consumption will deadlock because sub-sequence completion depends on other sub-sequences being consumed concurrently.