You failed to change "Tax Category" in app "Manage G/L Account Master Data". Error "The data contains error, you cannot save" occurs.
SAP S/4HANA Cloud 1902
Reproducing the Issue
In app "Manage G/L Account Master Data", try to change field "Tax Category" for a G/L account.
If the G/L account has a balance, then it is correct behavior that "Tax Category" cannot be changed.
You need to remove the balance by generating a posting against this account, then field "Tax Category" can be changed.
Fiori App "Manage G/L Account Master Data"
""Tax Category", change, "Manage G/L Account Master Data", Cloud , KBA , FI-FIO-GL , Fiori UI for General Ledger Accounting , Problem