Sales are not for that ECR

If you use

several cash registers,
several enterprises,
bind the cash register to the company in the settings,
but print a receipt "at will" and select an enterprise when paying,

then the reason may be a conflict of conditions that contradict each other.

If the "optional" check printing setting is enabled, but in Settings → Options → Check → Fiscal printer you select a ECR for an enterprise, this enterprise will be used to print a check.

The selection of the company on the Payment form will work when the Mandatory receipt printing for each sale setting is enabled.