QB Issue Resolution:

You can get around this new (helpful?) import feature by clicking the little link on the import dialog. See BRC – QuickBooks 2019 IIF Import Changes

Among other new helpful behaviors, QB now balks if you use any unknown field names in the ! definition rows. I do not know why QB thinks this is helpful, as QB has always just ignored fields it doesn’t understand, which has worked fine for 25 years. Until now.
The issue in your case is that TRANSID is not a valid field name while TRNSID is.
This has always been the case, but in the past you just didn’t notice. It will work fine, probably better, to change this for all clients.

Resolution for Issue '[ERROR] TRANSID is not a valid column name for record CHECK. [15101]' available: Yes (Solved).
Source: Intuit Community forum.

[ERROR] TRANSID is not a valid column name for record CHECK. [15101]: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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