I am changing company number for customer but I get message "Company not found" Yet, I can see the company number when I zoom out to find it.

tccom0500s000 zoom shows the records of tccom001.

However in tccom1101m000 (customers) you only can select a finance company (defined in tfgld004.gcmp).

When you change the field tccom010.comp it goes to include itfgld4004.
The function company.allowed checks that the company can be selected in tfgld004.gcmp and there is a record in tfgld001 too.