Playing 2/1 Game Force, you cannot make a 2/1 response with invitational values. If you hold a hand like ♠7 ♥943 ♦A52 ♣KQJ874 and your partner opens 1♠, you cannot make a game forcing 2♣ response. If you were playing Standard American, you could comfortably bid 2♣ to show an invitational or better hand. Invitational one-suited hands can be difficult to show playing 2/1 Game Force.
On the auction to the right, should South's 3♣ jump rebid show a hand like
(a) ♠7 ♥943 ♦A52 ♣KQJ874 or a hand like
(b) ♠7 ♥743 ♦52 ♣KQJ9874
If North had a hand like ♠AQ852 ♥AQ ♦QJT8 ♣65, you would want to bid 3NT opposite hand 1 and pass 3♣ opposite hand 2. Both hand 1 and hand 2 have legitimate reasons for bidding 3♣. If North has to guess which hand South has, it will be painful when North makes the wrong guess. You need different sequences to show an invitational one suited hand versus a hand which is simply trying to improve the contract.
Avoid guessing by using an immediate invitational jump shift to show a 1-suited invitational hand. With that agreement, South responds 3♣ immediately to show a long, strong club suit which can provide a source of tricks and an outside entry. With the weak hand with long clubs, South responds 1NT and then shows their club suit.
Invitational jump shifts are covered in the 2nd lesson in our 2/1 Game Force video lesson series.