In the Party configuration for the company, I tried to insert a real Swiss VAT number and selected the type “VAT”
Invalid VAT number "CHE-123.456.789" on party "foo"
If I change the type of the number to nothing, it accepts the number
Swiss VAT numbers have the format demonstrated above, CHE-123.456.789
As a workaround, I put the VAT number in the company footer for reports.