。
She might not be able to wait any longer.
Literal
She [topic-は] beyond-this wait-cannot [might-かもしれない].
Two patterns stacked. これ以上 ('any further, beyond this point') is a fixed expression for marking a limit being approached — usually paired with negative or limiting predicates: これ以上できない ('can't do any more'), これ以上は危険だ ('beyond this is dangerous'). 待てない is the negative potential of 待つ ('to wait') — 'cannot wait.' かもしれない ('might, may') hedges the whole thing as possibility rather than certainty. Together they describe someone at the edge of their patience: not yet broken, but close.