an argument or operand has the wrong type.

what it means

The formula asked for arithmetic or a function argument of one type and got another — usually text where a number was expected.

a formula that produces it

="a"+1

common causes

• arithmetic on text, e.g. ="a"+1
• a number stored as text, often after an import
• a function given a range where it expects a single value
• date arithmetic against something that is not a date

how to fix it

• convert with VALUE or DATEVALUE before calculating
• clean imported text with TRIM and CLEAN
• check each argument against the function reference signature

related

#NUM! · #NAME?