Formula picklist field issue

Detection

• String: Les champs de liste de sélection sont pris en charge uniquement dans certaines fonctions.

Resolution

You probably changed the type of a field that is used in a formula.
Update the formula to use a field compliant with formulas.
More details at https://help.salesforce.com/articleView?id=sf.tips_on_building_formulas.htm&type=5