Johann, Christophe has pointed you to the articles covering the options available in AC... Working Units and Dimension settings and styles.
The specific answer of being able to dynamically enter units in either metric or imperial while working is that it is not possible. You must switch the working units.
But there are some tricks that may help you, as a new user:
First, note that the Tracker (the pop-up data entry box) is customizable in the Work Environment.
Also, note that when drawing something with linear length, for example a Wall... tapping the "d" key highlights the distance (length) entry field of the tracker, immediately ready to enter the wall length. Or, you can change your Tracker settings (Work Environment) to 'prefer polar coordinates' - in which case the distance field is already highlighted, ready for you to simply type the length.
When Working Units are set to Imperial, there are shortcuts for data entry - especially useful if you have a numeric keypad on your keyboard. You do not need to use the feet (') and inches (") marks, although you can:
Examples:
10 is the same as entering 10'
10/ is the same as entering 10"
5-4 is 5'-4"
3/4 is 3/4"
AC has no punctuation that lets you enter metric while in imperial mode. (E.g., you can't enter 10mm)
When in metric mode... the imperial shortcuts (including ' and ") above are not allowed.
If you need to switch back and forth between metric and imperial working units often - you can minimize your pain by assigning a keyboard shortcut to the Working Units menu item. In the attached screenshot, I'm about to assign the 'u' key (for units?) to the Working Units dialog. Afterwards, one keystroke brings it up for switching. About the best I can think of if you have to go back and forth... Note: it is essential to click the "Assign" button before OK--ing out of the Working Environment > Keyboard Shortcuts.
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AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB