Sometimes after install some applications the USSD code has errors like this
Watch this video to fix it
:Or read this
A simple and effective way to fix Connection Problem or Invalid MMI Code on android phone is to add a comma at the end of the Prefix code. For example: If your Prefix code is *2904*7#, then adding the comma at the end would do just the trick which will look like *2904*7#, OR You can use the + symbol after * like *+2904*7# By adding the comma it forcecs the operation to execute and overlook any error that’s in the way.
Hopes that help you and solve your problem, be happy and smile ☺
