What I dDid Yesterday

Verbs in titles are capitalized typically

I worked out in my house with a pull up bar that is mounted on my patio's wall.

This is OK as it is, but it would be more normal to hear "on my patio wall."

I've been using a callisthenics app called Thenics, Iand it has all the tracking and routines that I need, without the high subscription fees that other apps charge.

I went to a friend's flat to build a PC, but we have a problem since the PC isn't turning on.

the "we have ...turning on" part is an independent clause, so it needs a coordinating conjunction in front of it to stay with the first part.

you could also say something like:

I went to a friend's flat to work on a PC that isn't turning on.

We're thinking that the motherboard is failing, since my friend received a crushed and open ated box without protection from the seller or carrier.

crushed is past tense, so "opened" should be in past tense as well