Decimal field validation in sql server

please advise on decimal field validation in sql server.

I pass decimal value as varchar to validate the length of parameter, when I pass a character in between I get error Error converting data type varchar to numeric which I should throw error as only numeric allowed.

declare @RequestAmount varchar(21) = '12a678'

IF isnumeric(@RequestAmount)=0 select 'has char' else select 'no char'

IF @RequestAmount like '%[a-Z]%' select 'has char' else select 'no char'