According to an announcement sent out by Ecocash Zimbabwe, ZETDC (the branch of ZESA that distributes and sells electricity) has raised the minimum transaction amount for ZESA token to $20 ZWL. This means that if you want to buy electricity units you have to spend at least $20 otherwise the transaction will fail.

Dear Valued Customers.

Please note that ZETDC has set the minimum purchase amount for electricity tokens at ZW$20 for all service providers effective 18 October 2019.
The minimum electricity token purchase on EcoCash has therefore been set at ZW$20.
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No one will lose sleep over this

ZESA recently hiked their tariffs in line with inflation. This means that almost everyone was spending way more than $20 if they wanted to buy a meaningful amount of electricity. The previous limit was $10 and its doubtful anyone was buying that little electricity anyway.