Declare Un-static Decimal Value

I try to declare a variable with un static decimal point in the following statement:-

declare @mpt int;
declare @mpq nvarchar (1)
SELECT @mpq = (SELECT mpq FROM ims.parm)

exec('declare @MM1 decimal (10,'+ @mpq +');')

set @mm1 = 1
print @mpt
when i print the declare statement looks like the following:
declare @MM1 decimal (10,2);
but with exec the statement, I have the following error:-
Msg 137, Level 15, State 1, Line 18
Must declare the scalar variable "@mm1".

Best regards