"To be able to write a message"

Translation: A putea scrie un mesaj

[FlorinGh.]

"a putea sa scrie un mesaj" should be accepted; this is actually more natural in RO

[iiai]

I could agree, but in your version someone might assume that we have the 3rd person singular (he, she), and this is a constraint. An example with this infinitive: ”A putea scrie un mesaj este o abilitate care se dobândește la școală.” Try to add your "să" here...

[vincemat]

"A putea să scrie un mesaj" was just accepted! 24 July 2020

[Victoria684052]

why not "A putea a scrie un mesaj"?

[MariaMyshk]

in what cases we have to use să between verbs? Is there any rule?

[relox84]

I'd say that you almost always use să + subjunctive between verbs, and putea is an exception: it can be followed by either the infinitive or să + subjunctive.