Decision Table status shows “Activation in Progress” and then reverts to “Inactive”

1. Run the query to check the RefreshFailureReason: Select Id, IsDeleted, DeveloperName, Status, RefreshFailureReason, Type, UsageType FROM DecisionTable.

 2. If the RefreshFailureReason shows 'common.exception.ApiException: Hash Key Group contains more than 50 rows', change the Decision Table priority from Medium to Low.

3. Change Decision Table priority: Change the Decision Table Type from Medium to Low, which resolves the activation issue. 

Currently, there is a limitation of MaxRowsPerHashKey in medium volume DTs(Advance DT) of 50. So check if the values of all the Required and Equals columns (if you keep Optional checkbox unchecked while defining input of DT, column becomes Required automatically) of more than 50 rows in sObject data should not be same. You can consider following alternatives

try using Low volume DT(Standard DT) , as Low volume DT does not have this limitation of MaxRowsPerHashKey. You can use Low volume DT , if number of rows in SObject is less than 25k (without group by ), you can also use Low volume DT with upto  1M SObject rows (with groupBy usage).

Refer to the documentation for additional details on Decision Table Limits: https://help.salesforce.com/s/articleView?id=ind.business_rules_engine_default_limits.htm&type=5.