@Carlos. 

Disable and enable the skill again in Alexa. Check the Kasa app and make sure the plug works in the app remotely, meaning your phone has the Wi-Fi disabled.

Another thing a user tried that others confirmed worked is below:

Login into alexa.amazon.com on your laptop, click on Skills on the left nav, scroll down to the TP-Link Kasa icon, click it and click on the DISABLE SKILL button at the top. Once the skill is disabled go back and re-enable it by clicking on Enable on the top right. Enter the TP-Link Cloud log in credentials during the enablement process. Add your plug if required and try ON/OFF functionality again within the Alexa app. If you're in an "Amazon Household" account say to Alexa ""switch to John's account"" or whomever's account the plug is registered to. Last week I must have changed the password with some other TP-Link service (Tp-Link forum login, repeater setup, or whatever) but plugs or app dashboards never threw visuals error and were linked to the tp cloud with an invalid older password.

This worked for me, after -months- of messing with it.

1) Find the least used channel from 1, 6 and 11 in your 2.4 settings. Use that. (6 is usually the most crowded, followed by 11 and 1, but you need to check actual use in your location.)

2) Unless you need 2.4 N, reduce the bandwidth from 40 to 20.

If that doesn't work, then...

3) make sure you are WPA2/WPA3 PERSONAL for your login, and make sure you use the same password on all your bands.

After making the changes, unplug from the wall socket all your wifi plugs, wait 20 seconds, and plug them back in again.

Good luck!