A formula has the wrong type of argument — often text where a number is expected.
Fix this with AI#VALUE! means Excel can't compute because an argument is the wrong data type — e.g. arithmetic on text, or a date stored as text.
Convert text to numbers
Use VALUE(), multiply by 1, or Text-to-Columns.
Clean the text
Apply TRIM and CLEAN to strip spaces and stray characters.
A cell in the range holds text. Convert it to a number or use SUM (which ignores text) on a clean range.