SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table TableName (availability VARCHAR(20) );
    ALTER TABLE TableName
    ADD CONSTRAINT availability_check CHECK (availability IN ('available', 'not available'));
INSERT INTO TableName(availability) VALUES('foo');
Msg 547 Level 16 State 0 Line 1
The INSERT statement conflicted with the CHECK constraint "availability_check". The conflict occurred in database "fiddle_239c5083e3ce46288302c122891454df", table "dbo.TableName", column 'availability'.
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.