Consideration about Salesforce Picklist field using in the Forumula field

if you refer in your fromula creation times as a Picklist field then you get the below error,

Error: Picklist fields are only supported in certain functions

so try to convert from picklist to TEXT and use it.

Picklist fields can only be used in the following functions:

• ISPICKVAL—Compares the value of a picklist to a single value.
• CASE—Compares the value of a picklist to multiple values.
• TEXT—Converts a picklist value into a text value so that you can work with the value in functions that support text value, such as CONTAINS. (Only available in formula fields, validation rules, and workflow field updates.)
• The TEXT function always returns picklist values in your organization’s master language, not the language of the current user.
• Multi-select picklist fields can only be used in the following functions:
  • INCLUDES
  • ISBLANK
  • ISNULL
  • ISCHANGED (Only in assignment rules, validation rules, workflow field updates, and workflow rules in which the evaluation criteria is set to Evaluate the rule when a record is: created, and every time it’s edited)
  • PRIORVALUE (Only in assignment rules, validation rules, workflow field updates, and workflow rules in which the evaluation criteria is set to Evaluate the rule when a record is: created, and every time it’s edited)

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