Non-supported type change for table?

[Astashonok]

I have a table where I want to change a column type to timestamp for field created_at:

define table zcustomer {
  key client : abap.clnt not null;
  key kunnr  : char10 not null;
  key name1  : char20 not null;
  created_at    : tzntstmpl;
}

however on activation it throws error mentioned in the title

On the other hand when I create table with tzntstmpl field from scratch it goes fine. Where this error comes from?

I'm using trial environment of ABAP BTP.

[FrankJentsch]

If the respective database table in the dev system contains data, and if the intended change (according to the old and new data type) might lead to data loss e.g. due to length reduction or changes from char to numeric, a conversion is required.

There are two options:

• Use the quick fix in ADT in the database table editor to convert the table
• Remove the whole data (if possible if it is only test data in the dev system), because conversion of an empty table is always possible

[Astashonok]

so late answer but thanks anyway. It worth giving more descriptive error, like "type change is impossible due to existing data in table", "the change may incur table data loss". Such generic error gives little use