I try to use cash on delivery and / or check/money order , or even purchase order. None of them work. I can toggle the Enabled selection from No to Yes but then once saved it goes back to No. The page reports the configuration was saved but it just isn't, it's returned to previous setting.
Hope for help
Regarding your payment method issue, it seems there is some third-party module that is being conflicted. Kindly check that once disabling that.
I hope this will help you to solve your issue.
If not, feel free to contact us.
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php bin/magento setup:upgrade php bin/magento setup:di:compile php bin/magento setup:static-content:deploy -f php bin/magento indexer:reindex
and then try to save the configuration.