You have a range of numbers in Excel but there are some error cells. When you try to use AutoSum, it selects the wrong range. When you select the correct range, the Sum is replaced by an error.
This video shows two methods for solving the problem:
With Microsoft 365, add IFERROR inside the SUM.
With older Excel, use the AGGREGATE function to ignore errors.

This video answers these common search terms:
How to get excel to ignore error in sum.
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Table of Contents
(0:00) AutoSum with Error cells
(0:18) Add IFERROR inside of SUM
(0:46) Use AGGREGATE to ignore errors
(1:13) Wrap-u