Welcome to the forum!

There are several ways to enter tuplets (Simple Entry Tool, Speedy Entry Tool, computer keyboard, mouse, MIDI keyboard).

My guess is that you are using the Simple Entry Tool.

Ending a measure with a tuplet can be a problem, depending on the way you enter tuplets.

If you do not want to try one of the other ways to enter a tuplet, then I suggest that you go to

Simple menu > Simple Entry Options

and de-select {Check for Extra Notes}.

No problem, here. Enter your quarter note. Enter your first 16th, and go to the tuplet definition dialogue box. Set it as 18 sixteenths in the space of 12 sixteenths. OK, to return to the score. Enter the remaining 16ths.

If you give us more details about how you enter notes, and if you are on Mac or Windows, we may be able to give more advice. But one simple one: Don't use the tuplet tool! (And just for the record, I have Check for Extra Notes selected.)

Pigggy-backing on Peters comment, I long ago left Check For Extra Notes off in Simple and on in Speedy Entry as my default, not because of tuplets per se, but because it seemed to fit the way I work.