@faruk1 - the Customer data type is a special type of LookUp column (a polymorphic lookup), you cannot obtain a specific attribute from that column by using the usual dot notation.
The record you're trying to return an output for can refer to more than one table. Use the IsType and AsType functions to identify the right Table to return the result you need.
For example, assuming the name of your Customer LookUp column is "Direct Customer", you can use:
If(
IsType(
ThisItem.'Direct Customer',
Accounts
),
AsType(
ThisItem.'Direct Customer',
Accounts
).'Account Name', //change to display the column you need
AsType(
ThisItem.'Direct Customer',
Contacts
).'Full Name' //change to display the column you need
)
