To meet your requirement, make sure the choice column "Opportunity Stage" has single selection, then go to list settings>validation settings under General Settings>Set formula something like:
=IF([Opportunity Stage]="Closed",IF(ISBLANK([Contract Execution Date]),FALSE,TRUE),TRUE)
Result for your reference:
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.