Apart from the other answers, also verify that your sales cart rule indexer is set to "update on save" - M2 2.3.5
magento indexer:show-mode salesrule_rule
Sales Rule: Update on Save
If it is on "update on schedule", it will not work/apply on the cart page since that rule record not correctly added in
magento_salesrule_filter table. It will be fixed after the default schedule reindex of the
salesrule_rule. Otherwise, you may have to manually reindex the
magento indexer:reindex salesrule_rule
This scenario only occurred to newly created sales cart rules without using the coupon.
Line 171: ->setValidationFilter() - magento/codepool/vendor/magento/module-sales-rule/Model/Validator.php
Line 51: aroundSetValidationFilter() - magento/codepool/vendor/magento/module-advanced-sales-rule/Model/ResourceModel/Plugin/Rule/Collection.php
Hope this will help someone & happy code debugging!